Method for producing glass unit, pressure-sensitive adhesive sheet, and use of corrosion inhibitor

ABSTRACT

Provided is a method for producing a glass unit. The method comprises a step (A) of obtaining a glass plate comprising a glass substrate and a Low-E layer placed on the glass substrate; a step (B) of applying a protective sheet to the Low-E layer surface of the glass plate; an optional step (C) of subjecting the glass plate to at least one process selected from the group consisting of transportation, storage, processing, washing and handling; a step (D) of removing the protective sheet from the glass plate; and a step (E) of assembling a glass unit using the glass plate. The protective sheet comprises a substrate layer, and a PSA layer provided to at least one face of the substrate layer. The PSA layer comprises, as a corrosion inhibitor, a surfactant free of sulfur atoms.

CROSS-REFERENCE

The present application claims priority to U. S. Provisional PatentApplication No. 62/611,048 filed on Dec. 28, 2017, the entire contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method for producing a glass unit, apressure-sensitive adhesive sheet preferably used in the method, and useof a corrosion inhibitor in a pressure-sensitive adhesive sheet.

2. Description of the Related Art

In processing and transporting various articles, in known techniques toprevent damage (scratches, contamination, corrosion, etc.) of theirsurfaces, protective sheets are bonded to the surfaces for protection.The objects to be protected vary widely. For instance, protective sheetsare used on glass plates bearing Low-E (Low-Emissivity) layers as well.Low-E-layer-bearing glass plates are preferably used as buildingmaterials such as window glass because of the effects of the Low-Elayers to improve the efficiency to cool down and heat up indoor spaces.In producing such a glass plate, usually, until a Low-E-layer-bearingglass plate and another glass plate are assembled into a pair-glass(e.g. dual-pane glass) with the Low-E layer surface on the inside, aprotective sheet is applied via its adhesive layer to the Low-E layersurface which will be otherwise left exposed. This protects the Low-Elayer from damage, wearing, degradation, corrosion, etc. Conventionalart documents related to methods for producing glass units includeEP2150669B1 (Patent Document 1) and WO2016/139318A1 (Patent Document 2).

In these applications, as a removable bonding means, pressure-sensitiveadhesive (or PSA; the same applies hereinafter) can be preferably used.In general, pressure-sensitive adhesive has characteristics of being ina soft solid (viscoelastic) state in a room temperature range and easilyadhering to adherend under some pressure. A surface protective sheetusing PSA typically has a PSA layer on one face of a substrate sheetformed of a material such as resin and is constituted so as to achieve aprotection purpose when applied via the PSA layer to an adherend (anobject to be protected). Conventional art documents disclosing PSAsheets usable as surface protective sheets include JP2017-186517A(Patent Document 3), JP5719194B2 (Patent Document 4), JP2012-131976A(Patent Document 5) and JP3571460B2 (Patent Document 6). Patent Document3 discloses a surface protective sheet that protects the surface of ametal plate while it is being drawn. Patent Document 4 discloses asurface protective sheet for optical film such as polarizing plates.Patent Document 5 relates to a surface protective sheet for aself-cleaning hydrophilic coated plate. Patent Document 6 is directed toeasy removal of a surface protective sheet for metal plates on which topcoats are formed and to reduction of the degree of contamination causedby it.

SUMMARY OF THE INVENTION

PSA sheets used for surface protection are usually removed fromadherends when appropriate after serving protection purposes. Thus, thePSA is designed to have relatively low adhesive strength. Especially,with respect to an adherend having a large surface area, it is importantto limit the adhesive strength for avoiding slowing of release (heavypeel) and maintaining efficient removal. For instance, glass plates usedfor building materials such as window glass typified byLow-E-layer-bearing glass plates now undergo enlargement of surface areain view of efficiency of production, transportation, etc., andmainstream pieces now have surface widths of greater than 2.6 m, or even3 m or greater. Enlargement of surface area of adherend may lead toenlargement of adhesive area of the protective sheet (PSA sheet) andfurther to heavy peel. Thus, from the standpoint of the efficiency ofremoval, the adhesive strength is desirably limited to the minimum levelrequired.

The adhesive strength of the PSA as a viscoelastic body, however, can bemediated by chemical interactions with the adherend material and also bythe wetting properties relative to the adherend surface; and therefore,limited adhesive strength may indicate poorer wetting properties basedon the viscosity parameter (loss modulus) G″ of the PSA and even reducedtightness of adhesion to the adherend. The concept of tightness ofadhesion here is different from the concept of adhesiveness (adhesion)which indicates the strength of adhesion; it rather refers to a state ofair-tight (gap-free) contact between PSA and an adherend and to thelevel of tightness. With a PSA that provides insufficient tightness ofadhesion, minute open spaces exist between the adherend surface and thePSA; and through these spaces, corrosive components such as acid and abase may be carried by air or water to reach the protected surface. Someadherends to be protected include corrosion-sensitive substances such asmetal. For instance, because the Low-E layer usually includes a layer ofmetal such as silver, permeation of moisture and the like may causemetal corrosion. In the Low-E layer, the metal layer is covered with aprotection layer. However, it is difficult to completely eliminate arisk that the metal layer may come into contact with water through apinhole, etc. It has been desired to establish a means to preventcorrosion of an object being protected without impairing light peel,etc.

This invention has been made in view of these circumstances with anobjective to provide a method for producing a glass unit according towhich corrosion of a glass plate is prevented. Another objective of thisinvention is to provide a highly anti-corrosive PSA sheet. Yet anotherobjective of this invention is to provide use of a corrosion inhibitorthat can make a highly anti-corrosive PSA.

The present application provides a method for producing a glass unit.The method comprises a step (A) of obtaining a glass plate comprising aglass substrate and a Low-E layer (in other words, Low-E coating) placedon the glass substrate; a step (B) of applying a protective sheet to theLow-E layer surface of the glass plate; an optional step (C) of carryingout at least one process selected from the group consisting oftransportation, storage, processing, washing and handling; a step (D) ofremoving the protective sheet from the glass plate; and a step (E) ofassembling a glass unit using the glass plate. Here, the protectivesheet comprises a substrate layer and a PSA layer provided to at leastone face of the substrate layer. The PSA layer comprises a surfactantfree of sulfur atoms (or a sulfur-free surfactant) as the corrosioninhibitor.

According to this method, a glass plate is protected not only fromdamage, wearing, and degradation, but also from a risk of corrosion bythe presence of the protective sheet during transportation, storage,processing such as cutting, washing such as a water wash, various kindsof handling, etc. In particular, the sulfur-free surfactant in the PSAlayer of the protective sheet is free of sulfur which may cause metalcorrosion. The surfactant migrates from the interior of the PSA layer tothe adherend-side surface to fill and seal a space in the surface thatmay serve as a channel for corrosive component-containing water and air.In the glass unit production, this can prevent a glass plate coveredwith the PSA sheet from undergoing corrosion (typically metalcorrosion). According to this method, a glass unit can efficiently beproduced while corrosion of a glass plate is prevented.

In this description, the “water” includes moisture, steam, rain water,wash water, hot water, so-called moisture content as well as water (H₂O)as a compound that can be identified in any other form or by any othername. Thus, the “water” herein includes matter present as a liquid aswell as a gas (water steam) in the air, and also a mixture of these.

In a preferable embodiment, the glass plate has a width of 1 m orgreater. The width of the glass plate can be 2 m or greater, greaterthan 2.6 m, or even 3 m or greater. For a glass plate having such arelatively large surface area, the method disclosed herein can preventthe removability from degrading while providing excellentcorrosion-inhibiting effects. While not limited to an embodiment using aglass plate with such a large surface area, the step (B) may include astep of entirely covering one face of the glass plate with at least oneof the protective sheet.

In a preferable embodiment, the Low-E layer includes a metal layer. Morespecifically, the Low-E layer may include a silver layer. The thicknessof the Low-E layer may typically be 1000 nm or less. For a glass platehaving such a Low-E layer, the method disclosed herein is employed toeffectively prevent the metal from corroding.

In a preferable embodiment, the step (C) is essential and in the step(C), the glass plate is washed using water. The protective sheetprotecting the glass plate can prevent permeation of water, therebypreventing the occurrence of corrosion caused by water.

This description can also provide a method for protecting an articlesurface, the method including a step of applying a protective sheet tothe surface of the article before, during or after processing (anapplication step). Here, the surface or the interior of the articlecomprises a corrosion-sensitive substance. The protective sheetcomprises a substrate layer and a PSA layer provided to at least oneface of the substrate layer. The PSA layer further comprises, as thecorrosion inhibitor, a surfactant free of sulfur atoms.

The sulfur-free surfactant in the PSA layer does not contain sulfurwhich causes metal corrosion. The surfactant migrates from the interiorof the PSA layer to the adherend-side surface to fill and seal a spacein the surface that may serve as a channel for corrosivecomponent-containing water and air. This can prevent the adherendsurface covered with the PSA sheet from undergoing corrosion (typicallymetal corrosion).

The “article” in this description encompasses not only a product, butalso a part (component) and an in-process material that is used only inthe production process of a product. The “article surface” may be theentire surface of the article or part of the article surface.

The surface protection method according to a preferable embodimentfurther includes a step of removing the protective sheet from thearticle (a removal step). Between the application step and the removalstep, it may optionally include at least one process selected from thegroup consisting of transporting, storing, processing, washing andhandling the article having the protective sheet applied thereon. Thisprotects the article from damage, wearing, degradation, corrosion, etc.,by the presence of the protective sheet during transportation, storage,processing such as cutting, washing such as a water wash and variouskinds of handling. In particular, since it prevents permeation of watersuch as moisture, the occurrence of corrosion caused by water can beprevented. Because the protective sheet is then efficiently removed fromthe adherend, it is not interfering.

In a preferable embodiment, the article comprises a glass substrate, anda coating layer that is placed on the glass substrate and comprises ametal layer. With respect to a surface formed of a coating layer thatincludes a metal layer, the use of the protective sheet disclosed hereincan effectively inhibit corrosion of the metal.

In a preferable embodiment, the article is a glass plate having a Low-Elayer on the one face. The application step includes a step of applyingthe protective sheet to the Low-E-layer-bearing face of the glass plate.The surface protection method disclosed herein is preferably applied toa glass plate surface, in particular, a Low-E layer surface of a glassplate.

In a preferable embodiment, the glass plate has a width of 1 m orgreater. The application step includes a step of entirely covering oneface of the glass plate with at least one of the protective sheet. For aglass plate having such a relatively large surface area, the surfaceprotection method disclosed herein can be employed to preferably preventthe removability from degrading while providing excellentcorrosion-inhibiting effects.

This description provides a PSA sheet comprising a substrate layer and aPSA layer provided to at least one face of the substrate layer. The PSAlayer comprises a surfactant free of sulfur atoms as a corrosioninhibitor. With the use of the PSA comprising a sulfur-free surfactantas the corrosion inhibitor, the PSA sheet can bring about excellentcorrosion-inhibiting effects without sacrificing light peel forinstance.

In a preferable embodiment of the art disclosed herein (including theglass unit production method, the surface protection method, the PSAsheet, surface protective sheet and the use of corrosion inhibitor; thesame applies hereinafter), the corrosion inhibitor is an anionicsurfactant or a nonionic surfactant. The corrosion inhibitor ispreferably a phosphate represented by a formula (a):

; (in the formula (a), R¹ is —OH or —(OCH₂CH₂)_(n)OR³; R² represents—(OCH₂CH₂)_(m)OR⁴; n and m are identical or different and each is aninteger between 1 and 30; R³ and R⁴ are identical or different and eachis an organic group with 1 to 30 carbon atoms). The use of a phosphatehaving such a structure as the corrosive inhibitor, excellentcorrosion-inhibiting effects can be preferably obtained.

In a preferable embodiment of the art disclosed herein, the corrosioninhibitor content in the PSA layer is 0.05 part to 10 parts by weight to100 parts by weight of the base polymer in the PSA layer. When thecorrosion inhibitor content is in this range, excellentcorrosion-inhibiting effects can be achieved while limiting itsinfluence on the adhesive properties.

In a preferable embodiment of the art disclosed herein, the PSA layerhas a surface hardness of 0.3 MPa or greater. With the use of a specificcorrosion inhibitor, the PSA sheet disclosed herein can bring aboutcorrosion-inhibiting effects regardless of the adhesive properties.Thus, excellent corrosion-inhibiting effects can be obtained even in anembodiment where the surface hardness is at or above the prescribedvalue while the aged adhesive strength is suppressed.

In a preferable embodiment of the art disclosed herein, the PSA layerhas a surface hardness of 0.5 MPa or less. With the use of the PSA layerhaving a surface hardness of 0.5 MPa or less, the PSA sheet adherestightly to the adherend surface. The adhesion-tightening effects basedon this PSA's property are combined with the effect of the use ofcorrosion inhibitor to achieve greater corrosion inhibition.

In a preferable embodiment of the art disclosed herein, the PSA layer isformed from a water-dispersed PSA composition, a solvent-based PSAcomposition or a hot-melt PSA composition. The PSA layer disclosedherein can be favorably prepared by using these PSA compositions. In amore preferable embodiment, the PSA layer is formed from awater-dispersed PSA composition. The water-dispersed PSA tends to showgreater hydrophilicity than other forms of PSA. The water-dispersed PSAcomposition typically comprises an emulsifier. The presence of ahydrophilic region in the emulsifier is likely to allow penetration ofmoisture, etc. In such a PSA (typically a PSA comprising an emulsifier),the use of the corrosion inhibitor disclosed herein can providepreferable effects.

The emulsifier in the PSA according to a favorable embodiment is areactive emulsifier having a radically-polymerizable group. The reactiveemulsifier is integrated into the backbone of the base polymer byradical polymerization. Thus, the migration of the emulsifier is limitedwithin the PSA layer and the influence of the emulsifier is reduced atthe PSA layer surface.

In a preferable embodiment of the art disclosed herein, the PSA layer isan acrylic PSA layer comprising an acrylic polymer as its base polymeror a rubber-based PSA layer comprising a rubber-based polymer as itsbase polymer. With the use of the acrylic polymer or the rubber-basedpolymer as the base polymer, a PSA layer suited to the protectionpurposes can be preferably prepared. The base polymer may be chemicallyor physically crosslinked. The base polymer may be an acrylic polymercrosslinked by a crosslinking agent. The crosslinking agent may be atleast one species selected from the group consisting of anisocyanate-based crosslinking agent, an epoxy-based crosslinking agent,an oxazoline-based crosslinking agent and an aziridine-basedcrosslinking agent.

In a preferable embodiment of the art disclosed herein, the initial peelstrength to a glass plate is 5 N/20 mm or less. With the initial peelstrength of 5 N/20 mm or less, it tends to allow easy removal, easyre-application and efficient removal. In particular, when the PSA sheetdisclosed herein is used in an embodiment where it is applied to anadherend having a large surface area, in view of the efficiency ofremoval, it is desirable that the initial peel strength is limited to orbelow a certain value. For an adherend having such a large surface area,the use of the PSA sheet according to a preferable embodiment disclosedherein can preferably achieve both light peel and corrosion inhibition.

In a preferable embodiment of the art disclosed herein, the sulfurcontent in the PSA layer is less than 2000 ppm. By minimizing the sulfurcontent in the PSA layer, a level of corrosion inhibition can beprovided to the object being protected.

In a preferable embodiment of the art disclosed herein, the substratelayer is resin film. Non-porous resin film that does not allow passageof water in the thickness direction is favorable as the protective sheetmaterial. Resin film with a certain level of durability can protect theadherend from various types of external force that may cause damage,degradation, wearing, etc. The substrate layer preferably includes apolyolefinic resin layer, a polyester-based resin layer or a polyvinylchloride-based resin layer. The PSA sheet formed from these materials islikely to combine conformability to and protection of the adherend. Fromthe standpoint of the conformability, the substrate layer preferably hasa thickness up to 150 μm. Excellent conformability to adherend may besignificant in obtaining tighter adhesion.

The PSA sheet disclosed herein is highly anti-corrosive. Thus, it ispreferably used as a surface protective sheet for an article whosesurface or interior comprises a corrosive substance. The articletypically comprises a glass substrate and a coating layer that is placedon the glass substrate and includes a metal layer. More specifically,the glass plate may have a Low-E layer on one face. The Low-E layerusually includes a layer of metal such as silver. To protect such aLow-E layer from metal corrosion, the PSA sheet disclosed herein ispreferably used. This can protect the Low-E layer not only from damage,degradation and wearing, but also from corrosion. Thus, this descriptionprovides a surface protective sheet comprising the PSA sheet disclosedherein, and this PSA sheet preferably comprises the PSA layer providedto one face of the substrate layer. In view of the efficiency ofremoval, the PSA sheet disclosed herein is preferably used on thesurface of an adherend having a large surface area where the peelstrength tends to be limited. The PSA sheet disclosed herein ispreferably used in an embodiment where it covers the entire surface ofone face of a flat plate (favorably a flat plate having a smoothsurface, typically a glass plate having a Low-E layer) having a width of1 m or greater, for instance, 2 m or greater (or even 3 m or greater).

This description provides use of a surfactant free of sulfur atoms as acorrosion inhibitor in PSA. The fact that a sulfur-free surfactant inPSA functions as a corrosion inhibitor has been elucidated from studiesby the present inventors. According to the finding of this new function,excellent corrosion inhibition is obtained. In typical, the corrosioninhibitor may exist on the surface of an area in which the objectprotected from corrosion is present. More specifically, the corrosioninhibitor disclosed herein may exist at the interface between thesurface of this area and the PSA surface. The object protected fromcorrosion may be typically metal (e.g. silver).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) to 1(E) show a schematic diagram illustrating an embodimentof the glass unit production method.

FIG. 2 shows a cross-sectional diagram schematically illustrating anembodiment of the surface protective sheet.

FIG. 3 shows a schematic plot illustrating a loading-unloading curve bynanoindentation.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention are described below.Matters necessary to practice this invention other than thosespecifically referred to in this description can be understood by aperson skilled in the art based on the disclosure about implementing theinvention in this description and common technical knowledge at the timeof application. The present invention can be practiced based on thecontents disclosed in this description and common technical knowledge inthe subject field.

<Glass Unit Production Method>

The glass unit production method disclosed herein comprises a step (A)of obtaining a glass plate comprising a glass substrate and a Low-Elayer (in other words, Low-E coating) placed on the glass substrate; astep (B) of applying a protective sheet to the Low-E layer surface ofthe glass plate; an optional step (C) of subjecting the glass plate withthe protective sheet adhered thereon to at least one process selectedfrom the group consisting of transportation, storage, processing,washing and handling; a step (D) of removing the protective sheet fromthe glass plate; and a step (E) of assembling a glass unit using theglass plate. In this method, the PSA sheet disclosed herein is used as aprotective sheet (or a surface protective sheet). Further description isprovided below while referring to FIGS. 1(A)-1(E).

First, in the step (A), as shown in FIG. 1(A), a glass plate 100 havinga glass substrate 110 and a Low-E layer 120 placed on the glasssubstrate 110 (which may be referred to as a “Low-E-layer-bearing glassplate” hereinafter) is obtained. Low-E-layer-bearing glass plate 100 isobtained by forming Low-E layer 120 on one face of glass substrate 110.The Low-E layer comprises a metal layer, an oxide layer such as a metaloxide layer, and a nitride layer such as silicon nitride; usually has amulti-layer structure; and is formed by a known or conventionally-usedmethod such as spattering. The materials forming the respective layersin the Low-E layer include TiO₂, ZrO₂, Si_(X)N_(Y), ZnO_(X), Ag,NiCrO_(X), and SnO₂. As an infrared-reflective layer, a Ag layer ispreferably used. In the Low-E layer according to an embodiment, the Aglayer is typically present between ZnO_(X) layers. Depending on therequired properties, the Low-E layer may have a multi-layer structurewith 5 or more layers, for instance, 10 or more layers, or even 15 ormore layers. The thickness of each layer is not particularly limited. Itis usually 0 to 1000 Å, or suitably about 10 Å to 700 Å, for instance,about 30 Å to 300 Å. The thickness (overall thickness) of the Low-Elayer can be about 10 nm to 1000 nm (e.g. about 50 nm to 500 nm). Thesize of the glass substrate is not particularly limited with one side(width) being, for instance, about 1 m or greater, or about 2 m orgreater. Lately, pieces having surface areas as large as or larger than2.6 m or even about 3 m or greater at one side are used.

In the step (B), as shown in FIG. 1(B), a protective sheet 200 isapplied to the surface of Low-E layer 120 formed on glass substrate 110.Protection sheet 200 is typically applied to the surface in a removablemanner. Here, “(to be) applied in a removable manner” means adhesionwhose eventual release is intended or expected; in many cases, it refersto adhesion such that the adherend can maintain its surface conditionprior to the adhesion basically intact after the protective sheet (PSAsheet) is removed. From the standpoint of the protection, the size ofprotective sheet 200 is preferably about the same as the surface ofLow-E layer 120. Two or more protective sheets may be partially layeredto cover the surface to be protected. The material of the outermost face(the surface to which the protective sheet is applied) of the Low-Elayer is often an oxide such as TiO₂, ZrO₂, ZnO_(X), NiCrO_(X), andSnO₂; or a nitride such as Si_(Z)N_(Y). The material of the outermostface of the Low-E layer is usually not a metal such as Ag. In the Low-Elayer, the metal layer such as the Ag layer is present covered withprotective layers such as ZnO_(X) layers as described earlier. Becausethe metal layer may communicate with the outside through a pinhole orthe like and undergo corrosion, it is important to provide covering withthe protective sheet disclosed herein.

After the step (B), as the step (C), with respect to theLow-E-layer-bearing glass plate having the protective sheet appliedthereon, at least one process may be optionally carried out, selectedfrom the group consisting of transportation, storage, processing,washing and handling. The processing may be the sort of cutting and edgeseaming of the Low-E-bearing glass plate having the protective sheetapplied thereon. The cutting means and the cut size are suitablyselected in accordance with the purpose and are not particularlylimited. The protective sheet may be left on the Low-E layer surfaceeven after the Low-E-layer-bearing glass plate is cut. The cut glassplate is typically washed with water, etc. In the washing step, inaddition to the water, a detergent (including surfactant) may beoptionally used. During the transportation, storage, processing such ascutting, washing such as a water wash and various kinds of handling, asshown in FIG. 1(C), by the presence of protective sheet 200 thereon, theLow-E layer 120 is protected from damage, wearing, degradation,corrosion, etc.

Subsequently, in the step (D), protective sheet 200 is removed fromglass plate 100 (FIG. 1(D)). The PSA sheet used as protective sheet 200is removed from glass plate 100 (adherend) after achieving theprotection purpose. Glass plate 100 from which protective sheet 200 isremoved is usually heated and annealed in an oven. Subsequently, asshown in FIG. 1(E), using the glass plate 100, a glass unit 300 isfabricated (the step (E)). Glass unit 300 is typically a heat-blockingor thermally-insulating glass unit, which can be fabricated by obtaininga pair of glass plates of which at least one is a Low-E-layer-bearingglass plate and assembling a pair-glass (e.g. dual-pane glass) with thesurface of Low-E layer 120 of Low-E-layer-bearing glass plate 100 on theinside. Numbers 320 and 340 in FIG. 1(E) represent another glass plateforming the glass unit 300 and a spacer, respectively. Spacer 340 isplaced between glass plate 100 and another glass plate 320 to create anopen space between glass plates 100 and 320. In the method disclosedherein, in addition to the protective sheet, known orconventionally-used powder or coating liquid may be used together.

<Surface Protection Method>

The surface protection method disclosed herein uses the PSA sheetdisclosed herein as a protective sheet (or a surface protective sheet)and typically relates to a method by which surfaces of various articlesare partially or entirely protected, with the articles includingproducts, parts (components) and in-process materials used only inproduction processes. The protection method disclosed herein ischaracterized by comprising a step of applying the protective sheet tothe surface of an article before, during or after processing (anapplication step).

The surface protection method disclosed herein may further comprise astep of removing the protective sheet from the article (a removal step).Between the application step and the removal step, for the articlehaving the protective sheet applied thereon, the method may optionallyinclude at least one process selected from the group consisting oftransporting, storing, processing, washing and handling.

A favorable example of the surface protection method disclosed herein isas described earlier regarding the glass unit production method; theapplication step and the removal step of this method correspond to thesteps (B) and (D) in the production method, respectively. The object inthe surface protection method disclosed herein is not limited to theaforementioned Low-E-layer-bearing glass plate and can be applied tovarious types of articles. In particular, the object can be an articlethat comprises a corrosion-sensitive substance in its surface orinterior. The corrosion-sensitive substance is an object to be protectedfrom corrosion in the art disclosed herein and is a substance thatundergoes corrosion (typically metal corrosion) in the presence ofwater, an acid, a base, etc. The corrosion-sensitive substance istypically a metal such as silver, copper, and iron. Thecorrosion-sensitive substance (typically a metal) in the interior of thearticle may be present covered with a protection layer, etc. However,because it may communicate with the outside through a pinhole and thelike, it makes an object to be protected in the surface protectionmethod disclosed herein. A specific object to be protected may comprisea glass substrate and a coating layer that is placed on the glasssubstrate and includes a metal layer. Other matters in the surfaceprotection method are not particularly limited. They can be understoodby an ordinarily-skilled person in consideration of the description ofthe glass unit production method. Thus, details are omitted here.

<Constitution of PSA Sheet>

As used herein, the term “PSA” refers to, as described earlier, amaterial that exists as a soft solid (a viscoelastic material) in a roomtemperature range and has a property to adhere easily to an adherendwith some pressure applied. As defined in C. A. Dahlquist, “Adhesion:Fundamental and Practice” (McLaren & Sons (1966), P. 143), the PSAreferred to herein is a material that has a property satisfying complextensile modulus E* (1 Hz)<10⁷ dyne/cm² (typically, a material thatexhibits the described characteristics at 25° C.). The concept of PSAsheet herein may encompass so-called PSA tape, PSA labels, PSA film,etc. The PSA sheet disclosed herein can be in a roll or in a flat sheet.Alternatively, the PSA sheet may be processed into various shapes.

The surface PSA sheet disclosed herein has a PSA layer on a substratelayer (support substrate). FIG. 2 shows a cross-sectional structure ofthe PSA sheet according to an embodiment. PSA sheet 10 comprises a PSAlayer 2 provided on the first face 1A of a substrate-layer sheet 1. Whenused, it is applied to an adherend over the face 2A of PSA layer 2. WhenPSA sheet 10 is used as a surface protective sheet, the face 2A of PSAlayer 2 is applied to an object to be protected. The back face 1B (onthe reverse side of the first face 1A) of substrate layer 1 is also theback face of PSA sheet 10, forming the outer surface of PSA sheet 10.Prior to use (i.e. before applied to the adherend), PSA sheet 10 can bein a form where the face 2A (adhesive face, i.e. the bonding surface tothe adherend) of PSA layer 2 is protected with a release liner (notshown in the drawing) having a release face at least on the PSA layerside. Alternatively, PSA sheet 10 may be in a form where, with thesecond surface (back face) 1B of substrate layer 1 being a release face,PSA sheet 10 is wound in a roll so that the back face comes in contactwith the PSA layer 2 to protect the surface (adhesive face) 2A. PSAsheet can be an on-substrate double-faced PSA sheet having a PSA layeron each face of the substrate layer.

As the release liner, commonly-used release paper and the like can beused without particular limitations. For instance, a release linerhaving a release layer on a surface of a liner substrate such as plasticfilm and paper, a release liner formed from a low-adhesive material suchas fluorinated polymer (polytetrafluoroethylene, etc.) and polyolefinicresin, and the like can be used. The release layer can be formed bysubjecting the liner substrate to surface treatment with various releaseagents including silicone-based, long-chain alkyl-based, and fluorinatedkinds as well as molybdenum sulfide.

The thickness of the PSA sheet disclosed herein is not particularlylimited. From the standpoint of the handling properties, the lightnessof weight, etc., it is usually suitably about 1000 μm or less (typicallyabout 300 μm or less, e.g. about 150 μm or less). In an embodiment, thethickness of the PSA sheet is preferably about 120 μm or less, morepreferably about 100 μm or less, yet more preferably about 75 μm orless, or possibly, for instance, less than 60 μm. The thickness of thePSA sheet can be typically greater than 20 μm, preferably greater than30 μm, or more preferably greater than 40 μm, for instance, greater than45 μm.

As used herein, the thickness of the PSA sheet includes the thicknessesof the PSA layer and the substrate layer, but excludes the thickness ofthe release liner.

The thickness of the substrate layer constituting the PSA sheetdisclosed herein is not particularly limited. The thickness of thesubstrate layer can be, for instance, about 800 μm or less (typicallyabout 250 μm or less). In an embodiment, the thickness of the substratelayer (typically, non-foamed resin film) is preferably about 150 μm orless, more preferably about 100 μm or less, or yet more preferably lessthan 65 μm, for instance, less than 55 μm. With decreasing thickness ofthe substrate layer, the PSA sheet tends to exhibit greaterconformability to the adherend shape while its lifting and peeling tendto be inhibited. From the standpoint of adherend protection and handlingproperties, etc., the thickness of the substrate layer can be typicallyabout 10 μm or greater, preferably about 25 μm or greater, morepreferably greater than about 30 μm or greater, or yet more preferablygreater than 40 μm, or possibly, for instance, greater than 45 μm.

No particular limitations are imposed on the thickness of the PSA layerconstituting the PSA sheet disclosed herein. From the standpoint ofpreventing adhesive transfer to the adherend, the thickness of the PSAlayer is usually about 50 μm or less, suitably about 30 μm or less,preferably about 15 μm or less, or more preferably about 8 μm or less(e.g. less than 6 μm). In another embodiment, from the standpoint of theease of removal, etc., the thickness of the PSA layer is suitably about5 μm or less, about 4 μm or less, or possibly, for instance, 3 μm orless. From the standpoint of the adhesion, the thickness of the PSAlayer is usually suitably about 0.5 μm or greater, preferably about 1 μmor greater, or more preferably greater than 2 μm. The thickness of thePSA layer may be greater than 3 μm, for instance, greater than 4 μm.

<PSA Layer>

The type of PSA forming the PSA layer disclosed herein is notparticularly limited. The PSA layer may be formed from a PSA compositioncomprising, as the base polymer (the primary component among thepolymers, i.e. a component accounting for 50% by weight or more), one,two or more species selected among various polymers (adhesive polymers),such as acrylic, polyester-based, urethane-based, polyether-based,rubber-based, silicone-based, polyamide-based, and fluorinated polymers.The art disclosed herein can be preferably made, for instance, as a PSAsheet comprising an acrylic PSA layer or a rubber-based PSA layer.

The “acrylic PSA layer” here refers to a PSA layer comprising an acrylicpolymer as the base polymer. Similarly, the “rubber-based PSA layer”refers to a PSA layer comprising a rubber-based polymer as the basepolymer. The “acrylic polymer” refers to a polymer whose primary monomer(the primary component among the monomers, i.e. a component thataccounts for 50% by weight or more of the total amount of the monomersforming the acrylic polymer) is a monomer having at least one(meth)acryloyl group per molecule. Such a monomer may be referred to asan “acrylic monomer” hereinafter. As used herein, the “(meth)acryloylgroup” comprehensively refers to acryloyl group and methacryloyl group.Similarly, the “(meth)acrylate” comprehensively refers to acrylate andmethacrylate.

(Acrylic Polymer)

A preferable example of the acrylic polymer is a polymer of a startingmonomer mixture that comprises an alkyl (meth)acrylate (or a monomer Ahereinafter) and may further comprise another monomer (or a monomer Bhereinafter) that is copolymerizable with the alkyl (meth)acrylate. Theacrylic polymer typically has a monomer unit composition correspondingto the monomer composition of the starting monomer mixture.

A preferable monomer A is an alkyl (meth)acrylate represented by thenext general formula (1):

CH₂═C(R¹)COOR²   (1)

Here, R¹ in the formula (1) is a hydrogen atom or a methyl group. R² isan alkyl group having 1 to 20 carbon atoms. Hereinafter, such a range ofthe number of carbon atoms may be indicated as “C₁₋₂₀.” From thestandpoint of the polymerization reactivity, polymerization stability,etc., an alkyl (meth)acrylate wherein R² is a C₁₋₁₆ alkyl group ispreferable, and an alkyl (meth)acrylate wherein R² is a C₁₋₁₂ (typicallyC₁₋₁₀, e.g. C₁₋₈) alkyl group is more preferable.

Examples of an alkyl (meth)acrylate with R² being a C₁₋₂₀ alkyl groupinclude methyl (meth)acrylate, ethyl (meth)acrylate, propyl(meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate,isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate,pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate,heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl(meth)acrylate, isooctyl (meth)acrylate, n-nonyl (meth)acrylate,isononyl (meth)acrylate, n-decyl (meth)acrylate, isodecyl(meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, tridecyl(meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate,hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl(meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, etc.These alkyl (meth)acrylates can be used solely as one species or in acombination of two or more species.

The art disclosed herein can be preferably implemented in an embodimentwhere the alkyl acrylate as the monomer A accounts for about 50% byweight or more (more preferably about 75% by weight or more, yet morepreferably about 90% by weight or more, e.g. about 95% by weight ormore) of the total monomer content. As the alkyl acrylate, an alkylacrylate with R² in the formula (1) being a C₁₋₂₀ alkyl group ispreferable and an alkyl acrylate with R² being a C₁₋₁₂ (more preferablyC₁₋₁₀, particularly preferably C₁₋₈) alkyl group is more preferable. Theart disclosed herein can also be preferably implemented in an embodimentwhere the alkyl acrylate has a C₂₋₈ (typically C₄₋₈) alkyl group for R²in the formula (1). For the alkyl acrylate, solely one species or acombination of two or more species can be used. When two or more speciesof alkyl acrylate are used, to adjust the acrylic polymer's Tg to themost adequate range, etc., an alkyl acrylate A1 with R² being a C₄₋₂₀(more preferably C₄₋₁₀, or yet more preferably C₄₋₈) alkyl group and analkyl acrylate A2 with R² being a C₁₋₃ (more preferably C₁₋₂, e.g. C₂)alkyl group can be used together. In this embodiment, the alkyl acrylateA1 to alkyl acrylate A2 weight ratio (A1:A2) is not particularlylimited. It is usually about 5:95 to 95:5, or suitably about 10:90 to90:10, for instance, about 15:85 to 85:15.

In a preferable embodiment, the monomers include one, two or morespecies of alkyl methacrylate as the monomer A. With the use of thealkyl methacrylate, the base polymer can be preferably designed so as toachieve desirable surface hardness for the PSA layer. As the alkylmethacrylate, an alkyl methacrylate with R² in the formula (1) being aC₁₋₁₀ alkyl group is preferable and an alkyl methacrylate with R² beinga C₁₋₄ (more preferably C₁ or C₂₋₄) alkyl group is more preferable. Thealkyl methacrylate can be preferably used in combination with an alkylacrylate. When an alkyl methacrylate and an alkyl acrylate are usedtogether, with one, two or more species of alkyl methacrylate (e.g. C₂₋₄alkyl methacrylate) having a weight C_(AM) and one, two or more speciesof alkyl acrylate having a weight C_(AA), their ratio (C_(AM):C_(AA)) isnot particularly limited. In an embodiment, it is usually about 1:9 to9:1, suitably about 2:8 to 8:2, preferably about 3:7 to 7:3, or morepreferably about 4:6 to 6:4. In another embodiment, the ratio of theweight C_(AM) of the alkyl methacrylate (e.g. C₁ alkyl methacrylate,i.e. methyl methacrylate (MMA)) in the total amount (C _(AM)+C_(AA)) ofthe alkyl (meth)acrylate is usually about 30% by weight or lower,suitably about 20% by weight or lower, preferably about 10% by weight orlower, or more preferably about 7% by weight or lower. On the otherhand, the lower limit is usually about 0.1% by weight or higher,suitably about 1% by weight or higher, or preferably about 2% by weightor higher (e.g. about 3% by weight or higher).

The art disclosed herein can be implemented in an embodiment where themonomers are essentially free of an alkyl methacrylate as the monomer A.In an embodiment using an alkyl methacrylate, it can be implemented, forinstance, in an embodiment free of a C₁₋₃ alkyl methacrylate (typicallyMMA).

Examples of compounds that can be used as the monomer B may includefunctional group-containing monomers such as those described below.These functional group-containing monomers may be useful for introducingcrosslinking points into the acrylic polymer or for increasing thecohesiveness of the acrylic polymer. Functional group-containingmonomers can be used solely as one species or in a combination of two ormore species.

Carboxy group-containing monomers: e.g. ethylenic unsaturatedmono-carboxylic acids such as acrylic acid (AA), methacrylic acid (MAA),crotonic acid, carboxyethyl (meth)acrylate, and carboxypentyl(meth)acrylate; ethylenic unsaturated dicarboxylic acids such asitaconic acid, maleic acid, fumaric acid, and citraconic acid;

Acid anhydride group-containing monomers: e.g. acid anhydrides of theethylenic unsaturated dicarboxylic acids such as maleic acid anhydrideand itaconic acid anhydride;

Hydroxy group-containing monomers: e.g. hydroxyalkyl (meth)acrylatessuch as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, and4-hydroxybutyl (meth)acrylate; unsaturated alcohols such as vinylalcohol, allyl alcohol, 2-hydroxyethyl vinyl ether, 4-hydroxybutyl vinylether, and diethylene glycol monovinyl ether;

Amide group-containing monomers: for example, (meth)acrylamide,N,N-dimethyl(meth)acrylamide, N-butyl(meth)acrylamide,N-methylol(meth)acrylamide, N-methylolpropane(meth)acrylamide,N-methoxymethyl(meth)acrylamide, N-butoxymethyl(meth)acrylamide;

Imide group-containing monomers: e.g. N-isopropylmaleimide,N-cyclohexylmaleimide, itaconimide;

Amino group-containing monomers: e.g. aminoethyl (meth)acrylate,N,N-dimethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, t-butylaminoethyl (meth)acrylate;

Epoxy group-containing monomers: e.g. glycidyl (meth)acrylate,methylglycidyl (meth)acrylate, allyl glycidyl ether;

Cyano group-containing monomers: e.g. acrylonitrile, methacrylonitrile;

Keto group-containing monomers: e.g. diacetone (meth)acrylamide,diacetone (meth)acrylate, vinyl methyl ketone, vinyl ethyl ketone, allylacetoacetate, vinyl acetoacetate;

Monomers having nitrogen atom-containing rings: e.g.N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine,N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperazine,N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, N-vinyloxazole,N-vinylmorpholine, N-vinylcaprolactam, N-(meth)acryloyl morpholine,N-(meth)acryloylpyrrolidone;

Alkoxysilyl group-containing monomers: e.g.(3-(meth)acryloxypropyl)trimethoxysilane,(3-(meth)acryloxypropyl)triethoxysilane,(3-(meth)acryloxypropyl)methyldimethoxysilane,(3-(meth)acryloxypropyl)methyldiethoxysilane.

Other examples of the compound that can be used as the monomer B includevinyl ester-based monomers such as vinyl acetate and vinyl propionate;aromatic vinyl compounds such as styrene, substituted styrenes(α-methylstyrene, etc.) and vinyltoluene; non-aromatic ring-containing(meth)acrylates such as cyclohexyl (meth)acrylate, t-butylcyclohexyl(meth)acrylate, cyclopentyl (meth)acrylate and isobornyl (meth)acrylate;aromatic ring-containing (meth)acrylates such as aryl (meth)acrylates(e.g. phenyl (meth)acrylate, benzyl (meth)acrylate), aryloxyalkyl(meth)acrylate (e.g. phenoxyethyl (meth)acrylate), arylalkyl(meth)acrylate (e.g. benzyl (meth)acrylate); olefnic monomers such asethylene, propylene, isoprene, butadiene and isobutylene;chlorine-containing monomers such as vinyl chloride and vinylidenechloride; isocyanate group-containing monomers such as2-(meth)acryloxyethylisocyanate; alkoxy group-containing monomers suchas methoxymethyl (meth)acrylate and ethoxyethyl (meth)acrylate; vinylether-based monomers such as methyl vinyl ether, ethyl vinyl ether andisobutyl vinyl ether. These can be used singly as one species or in acombination of two or more species.

Yet other examples of the compound that can be used as the monomer Binclude polyfunctional monomers. Specific examples of polyfunctionalmonomers include compounds having two or more (meth)acryloyl groups permolecule such as 1,6-hexanediol di(meth)acrylate, ethylene glycoldi(meth)acrylate, pentaerythritol di(meth)acrylate, trimethylolpropanetri(meth)acrylate, dipentaerythritol hexa(meth)acrylate and methylenebisacrylamide. Polyfunctional monomers can be used singly as one speciesor in a combination of two or more species. When using such apolyfunctional monomer, its amount used is not particularly limited. Itis usually suitably about 2% by weight or less (more preferably about 1%by weight or less) of the total monomer content.

The monomer A content in the total monomer content can be, but is notparticularly limited to, for instance, about 50% by weight or greater;it is suitably about 60% by weight or greater, preferably about 70% byweight or greater, more preferably about 80% by weight or greater, oryet more preferably about 85% by weight or greater. With the inclusionof the monomer A in a prescribed amount, a PSA sheet having desirableadhesive properties can be favorably obtained. The art disclosed hereincan be preferably implemented, for instance, in an embodiment where themonomer A content in the total monomer content is about 90% by weight orgreater. In an embodiment, the monomer A content can be about 95% byweight or greater, or even about 97% by weight or greater. In anembodiment using a monomer A and a monomer B together, from thestandpoint of suitably obtaining the effects of the monomer B, themonomer A content in the total monomer content can be, for instance,99.9% by weight or less; it is usually preferably 99.5% by weight orless, more preferably 99% by weight or less, or about 97% by weight orless (e.g. 95% by weight or less).

When an aforementioned functional group-containing monomer iscopolymerized in the acrylic polymer, the ratio of the functionalgroup-containing monomer to all the monomers forming the acrylic polymeris usually preferably about 0.1% by weight or higher (typically about0.5% by weight or higher, e.g. about 1% by weight or higher), andpreferably about 40% by weight or lower (typically about 30% by weightor lower, e.g. about 20% by weight or lower). For instance, when acarboxy group-containing monomer is copolymerized in the acrylicpolymer, from the standpoint of obtaining desirable cohesive strength,the ratio of the carboxy group-containing monomer to the total monomercontent is preferably about 1% by weight or higher, or more preferablyabout 1.5% by weight or higher (e.g. 3% by weight or higher). It ispreferably about 20% by weight or lower (preferably about 10% by weightor lower, typically about 7% by weight or lower, e.g. about 3% by weightor lower). When a hydroxy group-containing monomer is copolymerized withthe acrylic monomer, from the standpoint of obtaining desirable cohesivestrength, the ratio of the hydroxy group-containing monomer to the totalmonomer content is usually suitably about 0.001% by weight or higher(typically about 0.01% by weight or higher, e.g. about 0.1% by weight orhigher), preferably about 1% by weight or higher, or more preferablyabout 3% by weight or higher; it is preferably about 10% by weight orlower (typically about 7% by weight or lower, e.g. about 5% by weight orlower).

(Rubber-Based Polymer)

In another preferable embodiment, the PSA layer can be a rubber-basedPSA layer. Examples of the base polymer include natural rubber;styrene-butadiene rubber (SBR); polyisoprene; butene-based polymercomprising a butene (1-butene or cis- or trans-2-butene) and/or2-methylpropene (isobutylene) as the primary monomer(s); A-B-A blockcopolymer rubber and a hydrogenation product thereof, e.g.styrene-butadiene-styrene block copolymer rubber (SBS),styrene-isoprene-styrene block copolymer (SIS),styrene-isobutylene-styrene block copolymer rubber (SIBS),styrene-vinyl-isoprene-styrene block copolymers (SVIS), hydrogenated SBS(styrene-ethylene/butylene-styrene block copolymer (SEBS)), andhydrogenated SIS (styrene-ethylene-propylene-styrene block copolymers(SEPS)). These rubber-based polymers can be used singly as one speciesor in a combination of two or more species.

(Tg of Base Polymer)

The Tg value of the PSA layer's base polymer (an acrylic polymer in caseof an acrylic PSA layer being used) is not particularly limited in Tg.The Tg of the base polymer can be, for instance, about 0° C. or lower.In the PSA sheet according to a preferable embodiment, the base polymerof the PSA layer has a Tg of about -−5° C. or lower. According to a basepolymer having such a Tg, a PSA layer that tightly adheres to adherendcan be favorably formed. In an embodiment where the base polymer has aTg of about −15° C. or lower (more preferably about −20° C. or lower,e.g. about −25° C. or lower), greater effects can be obtained. In thePSA sheet according to another preferable embodiment, from thestandpoint of the adhesion to adherends, the base polymer of the PSAlayer has a Tg of about −35° C. or lower, more preferably about −40° C.or lower, or yet more preferably about −45° C. or lower (e.g. about −55°C. or lower). The Tg of the base polymer is usually suitably −70° C. orhigher. From the standpoint of the cohesion of the PSA, etc., it ispreferably about −65° C. or higher, more preferably about −50° C. orhigher, or yet more preferably about −35° C. or higher. The basepolymer's Tg can be adjusted by suitably changing the monomercomposition (i.e. the monomer species used in the synthesis of thepolymer and their ratio).

In the present description, the Tg of a polymer refers to the valuedetermined by the Fox equation based on the Tg values of homopolymers ofthe respective monomers forming the polymer and the weight fractions(copolymerization ratio by weight) of the monomers. As shown below, theFox equation is a relational expression between the Tg of a copolymerand glass transition temperatures Tgi of homopolymers of the respectivemonomers constituting the copolymer.

1/Tg=Σ(Wi/Tgi)

In the Fox equation, Tg represents the glass transition temperature(unit: K) of the copolymer, Wi the weight fraction (copolymerizationratio by weight) of a monomer i in the copolymer, and Tgi the glasstransition temperature (unit: K) of homopolymer of the monomer i.

For the glass transition temperatures of homopolymers used fordetermining the Tg, the values found in known documents are used. Forinstance, with respect to the monomers listed below, for the glasstransition temperatures of their homopolymers, the following values areused:

2-ethylhexyl acrylate −70° C. n-butyl acrylate −55° C. ethyl acrylate−20° C. methyl acrylate  8° C. n-butyl methacrylate  20° C. methylmethacrylate 105° C. 2-hydroxyethyl acrylate −15° C. 4-hydroxybutylacrylate −40° C. vinyl acetate  32° C. styrene 100° C. acrylic acid 106°C. methacrylic acid 228° C. acrylonitrile 104° C.

With respect to the Tg values of homopolymers other than the exampleslisted above, the values given in Polymer Handbook (3rd edition, JohnWiley & Sons, Inc., Year 1989) are used. With respect to a monomer forwhich two or more values are listed in the Polymer Handbook, the highestvalue is used. When no values are given in the Polymer Handbook, valuesobtained by the measurement method described in Japanese PatentApplication Publication No. 2007-51271 are used.

(Synthesis of Base Polymer)

The method for obtaining the base polymer (e.g. an acrylic polymer) isnot particularly limited. Known polymerization methods can be suitablyemployed, such as solution polymerization, emulsion polymerization, bulkpolymerization, and suspension polymerization. Alternatively, it is alsopossible to employ photopolymerization involving irradiation of lightsuch as UV (typically carried out in the presence of aphotopolymerization initiator) and active energy ray irradiationpolymerization such as radiation polymerization involving irradiation ofradioactive rays such as β rays and γ rays. As the monomer supply methodin solution polymerization and emulsion polymerization, a suitablemethod can be employed among the all-at-once method where all thestarting monomer mixture is supplied in one portion, gradual supplymethod, portion-wise supply method, etc. The polymerization temperaturecan be suitably selected in accordance with the monomer species, thesolvent species, and the polymerization initiator species used, etc. Thepolymerization temperature is usually suitably about 20° C. or higher,preferably about 40° C. or higher, more preferably about 50° C. orhigher; it can also be about 60° C. or higher, about 65° C. or higher,or even about 70° C. or higher. The polymerization temperature isusually suitably about 170° C. or lower (typically about 140° C. orlower), or preferably about 95° C. or lower (e.g. about 85° C. orlower). In emulsion polymerization, the polymerization temperature ispreferably about 95° C. or lower (e.g. about 85° C. or lower).

The solvent (polymerization solvent) used in solution polymerization canbe suitably selected among heretofore known organic solvents. Forinstance, it is preferable to use aromatic compounds (typically aromatichydrocarbons) such as toluene, acetic acid esters such as ethyl acetate,aliphatic or alicyclic hydrocarbons such as hexane and cyclohexane, andthe like.

The initiator used in the polymerization can be suitably selected amongknown or commonly-used polymerization initiators in accordance with themonomer species and the type of polymerization method. For instance,azo-based polymerization initiators can be preferably used, such as2,2′-azobisisobutyronitrile, 2,2′-azobis(2-methylpropionamidine)disulfate, and 2,2′-azobis(2-amidinopropane) dihydrochloride. Otherexamples of the polymerization initiator include persulfates such aspotassium persulfate and ammonium persulfate; peroxide-based initiatorssuch as benzoyl peroxide, t-butyl hydroperoxide, and hydrogen peroxide;substituted ethane-based initiators such as phenyl-substituted ethane;and aromatic carbonyl compounds. Yet other examples of thepolymerization initiator include redox initiators by the combination ofa peroxide and a reducing agent. Examples of the redox initiator includea combination of a peroxide (hydrogen peroxide, etc.) and ascorbic acid,a combination of a peroxide (hydrogen peroxide, etc.) and an iron(II)salt, and a combination of a persulfate salt and sodium hydrogensulfite. These polymerization initiators can be used singly as onespecies or in a combination of two or more species. The polymerizationinitiator can be used in a usual amount. For instance, it can beselected from a range of about 0.005 part to 1 part by weight (typicallyabout 0.01 part to 1 part by weight) to 100 parts by weight of the totalmonomer content.

The surfactant (emulsifier) used in emulsion polymerization is notparticularly limited. Commonly-known anionic surfactants, nonionicsurfactants and the like can be used. A surfactant having a radicallypolymerizable functional group can also be used. Hereinafter, thesurfactant having a radically polymerizable functional group is referredto as a reactive (polymerizing) surfactant. In contrast to this, ageneral surfactant free of a radically polymerizable functional groupmay be referred to as a non-reactive (non-polymerizing) surfactant. Forthe surfactant, solely one species or a combination of two or morespecies can be used. The amount of surfactant is usually preferablyabout 0.1 part by weight or greater (e.g. about 0.5 part by weight orgreater) to 100 parts by weight of the total monomer content; and it ispreferably about 10 parts by weight or less (e.g. about 5 parts byweight or less) to 100 parts by weight of the total monomer content.

Examples of the non-reactive surfactant include anionic emulsifiers suchas sodium lauryl sulfate, ammonium lauryl sulfate, sodium dodecylbenzene sulfonate, sodium polyoxyethylene lauryl sulfate, sodiumpolyoxyethylene alkyl ether sulfates, ammonium polyoxyethylene alkylphenyl ether sulfates, sodium polyoxyethylene alkyl phenyl ethersulfates, and sodium polyoxyethylene alkyl sulfosuccinates; andnon-ionic emulsifiers such as polyoxyethylene alkyl ethers,polyoxyethylene alkyl phenyl ethers, polyoxyethylene aliphatic acidesters, and polyoxyethylene-polyoxypropylene block polymers. When usinga non-reactive surfactant, from the standpoint of corrosion inhibition,it is preferable to use a non-reactive surfactant free of sulfur atoms(i.e. a sulfur-free non-reactive surfactant).

The reactive surfactant is not particularly limited as far as it has aradically polymerizable functional group. For instance, the reactivesurfactant may have a structure such that a radically polymerizablefunctional group is incorporated in an aforementioned anionic surfactantor nonionic surfactant. Examples of the radically polymerizablefunctional group include vinyl group, propenyl group, isopropenyl group,vinyl ether group (vinyloxy group), and allyl ether group (allyloxygroup). The concept of propenyl group referred to herein encompasses1-propenyl group (CH₃—CH═CH—) and 2-propenyl group (CH₂═CH—CH₂— whichmay be called allyl group).

Examples of an anionic reactive surfactant include polyoxyethylene(allyloxymethyl) alkyl ether sulfates (e.g. ammonium salts),polyoxyethylene nonyl propenyl phenyl ether sulfates (e.g. ammoniumsalts), alkyl allyl sulfosuccinates (e.g. sodium salts), methacryloxypolyoxypropylene sulfuric acid ester salts (e.g. sodium salts), andpolyoxyalkylene alkenyl ether sulfates (e.g. an ammonium salt having anisopropenyl group as the terminal alkenyl group). When the anionicreactive surfactant is forming a salt, the salt can be, for instance, ametal salt such as sodium salt or a non-metal salt such as ammonium saltand amine salt.

Examples of a nonionic reactive surfactant include polyoxyethylene nonylpropenyl phenyl ether.

Although no particular limitations are imposed, in some embodiments, areactive surfactant having an oxyethylene chain can be preferably used.The oxyethylene chain refers to a structure of repeating oxyethyleneunits, that is, a structural moiety represented by —(C₂H₄O)_(n)—, with nindicating the number of repeats of the oxyethylene unit. For instance,in a preferable reactive surfactant, the number of repeats, n, is about5 to 30 (e.g. 8 to 25).

From the standpoint of the polymerization stability during the emulsionpolymerization, in some embodiments, it is preferable to use a reactivesurfactant having a propenyl group. A preferable reactive surfactant hasa propenyl group and also an oxyethylene chain.

From the standpoint of the emulsifying ability, etc., in someembodiments, an anionic reactive surfactant (e.g. an anionic reactivesurfactant having an oxyethylene chain) can be preferably used. When theanionic reactive surfactant is in a salt form, as the salt, a non-metalsalt is preferable. In particular, an ammonium salt is preferable.

When using a nonionic reactive surfactant, more favorable results can beobtained by the combined use with other surfactant(s), such as ananionic reactive surfactant, anionic non-reactive surfactant andnonionic non-reactive surfactant.

By carrying out emulsion polymerization of the starting monomer mixturein the presence of a reactive surfactant having a radicallypolymerizable functional group, the reactive surfactant may undergo areaction to be incorporated into the acrylic polymer. The reactivesurfactant incorporated in the acrylic polymer is unlikely to bleed outto the PSA layer surface because its move within the PSA layer islimited. Accordingly, the use of the reactive surfactant can reducebleed-out of a low molecular weight compound to the PSA layer surface.This is preferable from the standpoint of the low-contaminatingproperties. From the standpoint of obtaining greater low-contaminatingproperties, it is preferable to apply an embodiment using solely areactive surfactant as the surfactant for emulsion polymerization.

In the emulsion polymerization, as necessary, various heretofore knownchain transfer agents (which can be considered also as a molecularweight-adjusting agent or polymerization degree-adjusting agent) can beused. For the chain transfer agent, solely one species or a combinationof two or more species can be used. As the chain transfer agent,mercaptans can be used, such as n-dodecyl mercaptan, t-dodecylmercaptan, and thioglycolic acid. Alternatively, from the standpoint ofobtaining a high level of corrosion inhibition, it is preferable toavoid use of a sulfur-based chain transfer agent containing sulfuratoms. In a preferable embodiment, in the polymerization, a chaintransfer agent free of sulfur atoms (a sulfur-free chain transfer agent)is used. Specific examples of the sulfur-free chain transfer agentinclude anilines such as N,N-dimethylaniline and N,N-diethylaniline;terpenoids such as α-pinene and terpinolene; styrenes such asα-methylstyrene and α-methylstyrene dimer; benzylidenyl group-bearingcompounds such as dibenzylidene acetone, cinnamyl alcohol and cinnamylaldehyde; hydroquinones such as hydroquinone and naphthohydroquinone;quinones such as benzoquinone and naphthoquinone; olefins such as2,3-dimethyl-2-butene, 1,5-cyclooctadiene and sorbic acid; alcohols suchas phenol, benzyl alcohol and allyl alcohol; and benzenes such asdiphenylbenzene and triphenylbenzene. When using a chain transfer agent,its amount can be, for instance, about 0.01 part to 1 part by weight to100 parts by weight of the total monomer content. The art disclosedherein can also be preferably practiced in an embodiment that uses nochain transfer agent.

<PSA Composition>

The PSA layer of the PSA sheet disclosed herein can be formed fromvarious forms of PSA compositions. Examples of the forms of PSAcompositions include a solvent-based PSA composition containing the PSA(adhesive component(s)) in an organic solvent, a water-dispersed PSAcomposition containing at least part of the PSA dispersed in an aqueoussolvent, an active energy ray-curable PSA composition formulated so asto cure with active energy rays such as UV rays and radioactive rays toform PSA, and a hot-melt PSA composition which is applied in the moltenstate by heating and forms PSA when it cools to near room temperature.

From the standpoint of reducing environmental stress, a water-dispersedPSA composition can be preferably used. A favorable example of thewater-dispersed PSA composition is a water-dispersed PSA composition (awater-dispersed acrylic PSA composition, typically an acrylic emulsionPSA composition) comprising an acrylic polymer as the base polymer. Fromthe standpoint of the adhesive properties, a solvent-based PSAcomposition is preferable. The hot-melt PSA composition requiring no useof a solvent is advantageous for its excellent handling properties inthe production process.

(Crosslinking Agent)

In the art disclosed herein, the PSA composition used to form the PSAlayer preferably comprises a crosslinking agent. With the use ofcrosslinking agent, the surface hardness of the PSA layer can besuitably adjusted. The type of crosslinking agent used is notparticularly limited and can be suitably selected from heretofore knowncrosslinking agents.

Specific examples of the crosslinking agent include oxazoline-basedcrosslinking agents, aziridine-based crosslinking agents,isocyanate-based crosslinking agents, epoxy-based crosslinking agents,melamine-based crosslinking agents, peroxide-based crosslinking agents,urea-based crosslinking agents, metal alkoxide-based crosslinkingagents, metal chelate-based crosslinking agents, metal salt-basedcrosslinking agents, carbodiimide-based crosslinking agents,hydrazine-based crosslinking agents, amine-based crosslinking agents,and silane coupling agents. These can be used solely as one species orin a combination of two or more species. For instance, it is preferableto use one, two or more species selected from a group consisting ofoxazoline-based crosslinking agents, aziridine-based crosslinkingagents, isocyanate-based crosslinking agents and epoxy-basedcrosslinking agents. In particular, an isocyanate-based crosslinkingagent and an epoxy-based crosslinking agent are preferable.

As the oxazoline-based crosslinking agent, a species having one or moreoxazoline groups per molecule can be used without particularlimitations. In the water-dispersed PSA composition, it is preferable touse a water-soluble or water-dispersible oxazoline-based crosslinkingagent.

The oxazoline group can be either 2-oxazoling group, 3-oxazoline groupor 4-oxazoline group. Usually, a 2-oxazoline group-containingoxazoline-based crosslinking agent can be preferably used. As theoxazoline-based crosslinking agent, a water-soluble copolymer or awater-dispersed copolymer can be used, which is obtained bycopolymerizing an addition-polymerizable oxazoline such as2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline,2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline,2-isopropenyl-4-methyl-2-oxazoline and 2-isopropenyl-5-ethyl-2-oxazolinewith other monomer(s).

Examples of a commercial oxazoline-based crosslinking agent includeproducts of Nippon Shokubai Co., Ltd., under trade names EPOCROS WS-500,EPOCROS WS-700, EPOCROS K-2010E, EPOCROS K-2020E and EPOCROS K-2030E.

Examples of the aziridine-based crosslinking agent includetrimethylolpropane tris[3-(1-aziridinyl)propionate] andtrimethylolpropane tris[3-(1-(2-methyl)aziridinylpropionate)].

As an example of the isocyanate-based crosslinking agent, a bifunctionalor higher polyfunctional isocyanate compound can be used. Examplesinclude aromatic isocyanates such as tolylene diisocyanates, xylylenediisocyanate, polymethylene polyphenyl diisocyanate,tris(p-isocyanatophenyl)thiophosphate, and diphenylmethane diisocyanate;alicyclic isocyanates such as isophorone diisocyanate; and aliphaticisocyanates such as hexamethylene diisocyanate. Commercial productsinclude isocyanate adducts such as trimethylolpropane/tolylenediisocyanate trimer adduct (trade name CORONATE L available from TosohCorporation), trimethylolpropane/hexamethylene diisocyanate trimeradduct (trade name CORONATE HL available from Tosoh Corporation) andhexamethylene diisocyanate isocyanurate (trade name CORONATE HXavailable from Tosoh Corporation). In the water-dispersed PSAcomposition, it is preferable to use an isocyanate-based crosslinkingagent that is soluble or dispersible in water. For instance, awater-soluble, water-dispersible or self-emulsifying isocyanate-basedcrosslinking agent can be preferably used. A so-called blockedisocyanate-based crosslinking agent can be preferably used as aisocyanate-based crosslinking agent.

As the epoxy-based crosslinking agent, a species having two or moreepoxy groups per molecule can be used without particular limitations. Anepoxy-based crosslinking agent having 3 to 5 epoxy groups per moleculeis preferable. In the water-dispersed PSA composition, it is preferableto use a water-soluble or water-dispersible epoxy-based crosslinkingagent.

Specific examples of the epoxy-based crosslinking agent includeN,N,N′,N′-tetraglycidyl-m-xylenediamine,1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, 1,6-hexanediol diglycidylether, polyethylene glycol diglycidyl ether, and polyglycerolpolyglycidyl ether.

Commercial epoxy-based crosslinking agents include products ofMitsubishi Gas Chemical Co., Inc., under trade names TETRAD-X andTETRAD-C, a product of DIC Corporation under trade name EPICLON CR-5L, aproduct of Nagase ChemteX Corporation under trade name DENACOL EX-512,and a product of Nissan Chemical Industries, Ltd., under trade nameTEPIC-G.

As the carbodiimide-based crosslinking agent, a low or high molecularweight compound having two or more carbodiimide groups can be used. In awater-dispersed PSA composition, it is preferable to use a water-solubleor water-dispersible carbodiimide-based crosslinking agent. Examples ofcommercial carbodiimide-based crosslinking agents include theCARBODILITE series such as the CARBODILITE V series (aqueous solutions)including CARBODILITE V-02, CARBODILITE V-02-L2, and CARBODILITE V-04;and the CARBODILITE E series (aqueous dispersions) including CARBODILITEE-01, CARBODILITE E-02, and CARBODILITE E-04 available from NisshinboHoldings, Inc.

In an embodiment using an epoxy-based crosslinking agent, its amountused is not particularly limited. For instance, the amount of theepoxy-based crosslinking agent is suitably, to 100 parts by weight ofthe base polymer (typically an acrylic polymer), about 10 parts byweight or less, or for instance, about 8 parts by weight or less. Fromthe standpoint of adjusting the PSA layer's surface hardness to increasethe tightness of adhesion to the adherend, it is preferably less thanabout 6 parts by weight, more preferably less than 5 parts by weight,yet more preferably less than 4 parts by weight, or particularlypreferably 3 parts by weight or less (e.g. 2 parts by weight or less, oreven 1.5 parts by weight or less). The lower limit of the amount ofepoxy-based crosslinking agent is not particularly limited. To 100 partsby weight of the base polymer (typically an acrylic polymer), it issuitably about 0.001 part by weight or greater. From the standpoint ofthe cohesion and light peel of the PSA, it is preferably about 0.01 partby weight or greater, more preferably about 0.1 part by weight orgreater, or yet more preferably about 1 part by weight or greater.According to the art disclosed herein, even with a PSA that adheresrelatively loosely to adherend, the use of the corrosion inhibitor canprevent corrosion of the adherend. Thus, the art disclosed herein can beimplemented in an embodiment where an epoxy-based crosslinking agent isused in an amount of about 3 parts by weight or more, for instance, 5parts by weight or more (typically more than 5 parts by weight, or even6 parts by weight or more) to 100 parts by weight of the base polymer(typically an acrylic polymer).

In an embodiment using an isocyanate-based crosslinking agent, itsamount used is not particularly limited. The amount of theisocyanate-based crosslinking agent used to 100 parts by weight of thebase polymer (typically an acrylic polymer) is suitably about 10 partsby weight or less, or for instance, about 8 parts by weight or less.From the standpoint of adjusting the PSA layer's surface hardness toincrease the tightness of adhesion to the adherend, it is preferablyabout 5 parts by weight or less, more preferably 4 parts by weight orless, or yet more preferably less than 3.5 parts by weight. The lowerlimit of the amount of isocyanate-based crosslinking agent is notparticularly limited. To 100 parts by weight of the base polymer(typically an acrylic polymer), it is suitably about 0.1 part by weightor greater, or for instance, 0.3 part by weight or greater. From thestandpoint of the cohesion and light peel of the PSA, it is preferablyabout 0.5 part by weight or greater, more preferably about 1 part byweight or greater (e.g. 2 parts by weight or greater). The use of theisocyanate-based crosslinking agent facilitates the anchoring to thesubstrate layer.

The crosslinking agent content (the total amount of crosslinking agent)in the PSA composition disclosed herein is not particularly limited andcan be suitably selected in view of the composition and the molecularweight of the base polymer so as to obtain favorable properties aftercrosslinked. While no particular limitations are imposed, the amount ofthe crosslinking agent used to 100 parts by weight of the base polymer(typically an acrylic polymer) is usually about 0.01 part by weight orgreater, suitably about 0.1 part by weight or greater, or preferablyabout 1 part by weight or greater (e.g. about 2 parts by weight orgreater). From the standpoint of the adhesion, etc., the amount of thecrosslinking agent is usually suitably about 15 parts by weight or less(preferably about 10 parts by weight or less, e.g. about 5 parts byweight or less) to 100 parts by weight of the base polymer.

(Corrosion Inhibitor)

The PSA layer disclosed herein is characterized by including asulfur-free surfactant as a corrosive inhibitor. The sulfur-freesurfactant in the PSA layer does not include sulfur which causes metalcorrosion. The sulfur-free surfactant is a non-reactive compound free ofa radically-polymerizable group which can be included in the backbone ofthe base polymer upon radical polymerization. Therefore, it migratesfrom the interior of the PSA layer to the adherend-side surface to filland seal a space in the surface that may serve as a channel forcorrosive component-containing water and air, thereby preventing theadherend surface covered with the PSA sheet from undergoing corrosion(typically metal corrosion).

The corrosion inhibitor disclosed herein can be any surfactant free ofsulfur atoms and is not otherwise limited. Typically, it is anon-reactive surfactant having a hydrophobic group and a hydrophilicgroup. Favorable examples thereof include various types of surfactantsuch as anionic surfactants and nonionic surfactants. Anionicsurfactants usable as the corrosion inhibitor include carboxylates,sulfonates, sulfates, and phosphates; nonionic surfactants include estertypes, ether types, ester-ether types, and aliphatic acid alkanolamidetypes having an amide bond between a hydrophobic group and a hydrophilicgroup; and cationic surfactants include amine salt types, and quaternaryammonium salt types. As the corrosion inhibitor, an aliphatic acid amidecan be used as well. For the corrosion inhibitor, solely one species ora combination of two or more species can be used. These sulfur-freesurfactants do not cause an increase in adhesive strength of the PSA. Ina preferable embodiment, the corrosion inhibitor can be preferably usedin an embodiment where it is added after the polymerization, that is, anembodiment where the corrosion inhibitor is added to the base polymersynthesized in advance. Alternatively, the corrosion inhibitor can beused during the polymerization.

In a preferable embodiment, as the corrosion inhibitor, a phosphate isused. Examples of the phosphate include alkyl phosphates such as laurylphosphate and a lauryl phosphate salt; and a phosphate having anoxyethylene chain and a salt thereof. The salt can be, for instance,sodium salts, potassium salts, barium salts and triethanolamine salts ofthese phosphates. In the following, unless otherwise noted, the“phosphate” includes a salt. A phosphate is effective in inhibitingcorrosion at the interface between the PSA layer and the adherendwithout accompanying an increase in adhesive strength; and therefore, itis preferably used in an embodiment where it is removed after servingthe protection purpose. In particular, as the corrosion inhibitordisclosed herein, it is more preferable to use a phosphate having anoxyethylene chain. The oxyethylene chain-containing phosphate cansuppress the increase in adhesive strength with aging (i.e. increase thestability of the adhesive strength) while preventing penetration ofcorrosive components such as water, an acid and a base at the interfacebetween the PSA layer and the adherend surface. Here, the oxyethylenechain refers to a structural moiety in chain form that includes at leastone ethylene oxide (EO) unit and may further include other oxyalkyleneunit(s) (e.g. oxyalkylene unit(s) with about 3 to 6 carbon atoms). Onefavorable example of the oxyethylene chain-containing phosphate is aphosphate having an oxyethylene chain formed with an EO unit or repeatsof this. For instance, a phosphate represented by the next generalformula (a) or a salt thereof can be preferably used as the corrosioninhibitor.

In the general formula (a), R¹ is —OH or —(OCH₂CH₂)_(n)OR³; R²represents —(OCH₂CH₂)_(m)OR⁴; n and m indicate the number of moles of EOadded. The number of moles of EO added, n, is an integer between 1 and30; it can be typically an integer between about 1 and 20, preferably aninteger between about 1 and 10, for instance, an integer between about 1and 8. The number of moles of EO added, n, is preferably an integerbetween about 1 and 6, or yet more preferably an integer between 1 and 4(e.g. 2 and 4). In the general formula (a), the number of moles of EOadded, m, can be typically about the same as the number of moles of EOadded for n; n and m may be identical or different. R³ and R⁴ aremono-valent organic groups (typically hydrocarbon groups); for instance,each can be individually a group selected among an alkyl group,cycloalkyl group, aryl group, alkylaryl group, and arylalkyl group. R³and R⁴ are preferably linear or branched alkyl groups, aryl groups oralkylaryl groups. R³ and R⁴ are individually an organic group with 1 to30 carbon atoms, or possibly an organic group with 6 or more (preferably8 or more, e.g. 11 or more) carbon atoms. In a preferable embodiment, R³and R⁴ can be organic groups with 20 or fewer or preferably 18 or fewer,for instance, 15 or fewer carbon atoms. The phosphate salts representedby the general formula (a) can be, for instance, sodium salts, potassiumsalts, barium salts and triethanolamine salts of these phosphates. Forthe phosphate, solely one species or a combination of two or morespecies can be used.

Examples of the phosphates include polyoxyethylene alkylphosphoric acidesters such as polyoxyethylene tridecyl ether phosphate, polyoxyethylenelauryl ether phosphate, and polyoxyethylene octadecyl ether phosphate;and polyoxyethylene alkyl aryl phosphoric acid esters such aspolyoxyethylene nonyl phenyl ether phosphate, polyoxyethylene octylphenyl ether phosphate, polyoxyethylene dinonyl phenyl ether phosphate,and polyoxyethylene dioctyl phenyl ether phosphate. In an embodiment, aphosphoric acid ester having a molecular weight of 150 to 5000 can bepreferably used.

The amount of the corrosion inhibitor can be, for instance, about 0.05part by weight or greater to 100 parts by weight of the base polymer(e.g. an acrylic polymer); it is usually preferably about 0.1 part byweight or greater, or more preferably about 0.3 part by weight orgreater (e.g. about 0.5 part by weight or greater). From the standpointof leaving a lower degree of contamination on the adherend surface, theamount of the corrosion inhibitor used to 100 parts by weight of thebase polymer (e.g. an acrylic polymer) is usually suitably about30 partsby weight or less (e.g. 20 parts by weight or less), preferably about 10parts by weight or less, more preferably about 5 parts by weight orless, yet more preferably about 3 parts by weight or less, orparticularly preferably about 2 parts by weight or less (e.g. 1 part byweight or less).

The PSA composition can comprise, as necessary, a known tackifier suchas a rosin-based tackifier, terpene-based tackifier andhydrocarbon-based tackifier. From the standpoint of avoiding anexcessive increase in peel strength, the amount of tackifier ispreferably about 5 parts by weight or less to 100 parts by weight of thebase polymer, or more preferably about 1 part by weight or less. For thePSA sheet disclosed herein, the adhesive strength can be effectivelycontrolled by the base polymer's composition and Tg as well as PSA's gelfraction, etc.; and therefore, the surface protective sheet can bepreferably made in an embodiment using no tackifier as well.

The PSA composition may comprise, as necessary, various optionaladditives generally known in the field of PSA compositions, such asviscosity-adjusting agent (viscosifier, etc.), crosslinking accelerator,plasticizer, softener, filler, anti-static agent, anti-aging agent,UV-absorber, antioxidant and photo-stabilizing agent. From thestandpoint of corrosion inhibition, it is preferable to use optionaladditives free of sulfur. The PSA layer disclosed herein may berelatively thin and can be formed in an embodiment essentially free of aviscosifier (e.g. an acrylic viscosifier comprising a carboxylic acid,etc.). As the optional additive, an optional polymer different from thebase polymer (e.g. an acrylic polymer) can be used as well. The optionalpolymer content is usually about 10% by weight or less (e.g. about 1% byweight or less) of the PSA composition. The PSA composition disclosedherein may be essentially free of the optional polymer. With respect tothese various optional additives, heretofore known species can be usedby typical methods. Because these additives do not characterize thepresent invention in particular, details are omitted.

(Formation of PSA Layer)

As for the method for providing the PSA layer to a support substratewhich forms the substrate layer, it is possible to employ a directmethod where the PSA composition as described above is directly provided(typically applied) to the support substrate and subjected to a curingtreatment; a transfer method where the PSA composition is applied to asuitable release face (e.g. a releasable surface of a transfer sheet)and subjected to a curing treatment to form a PSA layer on the surfacefollowed by applying and transferring the PSA layer to the supportsubstrate; and so on. The curing treatment may comprise one, two or moreprocesses selected among drying (heating), cooling, crosslinking,supplemental copolymerization reaction, aging, etc. The curing treatmentreferred to herein also encompasses, for instance, a process (heatingprocess, etc.) simply to allow a PSA composition containing a solvent todry, a process simply to cool down (solidify) a heat-melted PSAcomposition. When the curing treatment comprises two or more processes(e.g. drying and crosslinking), these processes may be performed at onceor stepwise.

The PSA composition can be applied, for instance, using a commonly usedcoater such as a gravure roll coater, reverse roll coater, kiss rollcoater, dip roll coater, bar coater, knife coater and spray coater. Fromthe standpoint of accelerating the crosslinking reaction, increasing theproductivity, etc., the PSA composition is preferably dried with heat.The drying temperature may vary depending on the object (a supportsubstrate, etc.) to which the PSA composition is applied, but it can be,for instance, about 40° C. to 150° C.

(Gel Fraction)

The weight fraction (gel fraction) of the ethyl acetate-insolubleportion of the PSA layer disclosed herein is not particularly limited.It can be, for instance, about 40% or higher (typically about 50% orhigher). In an embodiment, the gel fraction of the PSA layer is suitablyabout 60% or higher, preferably about 80% or higher, or more preferablyabout 90% or higher. The gel fraction of the PSA layer can be, forinstance, about 95% or higher (e.g. about 98% or higher). Withincreasing gel fraction, the cohesion of the PSA tends to increase whilethe aged adhesive strength tends to be suppressed. The maximum gelfraction is theoretically 100%. In some embodiments, the gel fractioncan be, for instance, about 98% or lower; it is suitably about 97% orlower; it can be about 95% or lower (e.g. about 90% or lower). The gelfraction can be adjusted by the selection of, for instance, the basepolymer composition, the polymerization method and conditions for thebase polymer, the molecular weight of the base polymer, the presence ofa crosslinking agent as well as its type and amount used if any, and soon. The gel fraction is determined by the method described below. Thesame method is used for the working examples described later.

(Degree of Swelling)

The degree of swelling of the PSA layer disclosed herein is notparticularly limited and can be usually about 30-fold or less. From thestandpoint of obtaining at least certain surface hardness, the degree ofswelling is suitably about 20-fold or less, preferably about 15-fold orless, or more preferably about 12-fold or less, for instance, about10-fold or less, or even about 8-fold or less. The minimum degree ofswelling is theoretically 1-fold; it can be usually about 3-fold orgreater, for instance, about 5-fold or greater, or suitably about 7-foldor greater. The degree of swelling can be adjusted, for instance,through the molecular weight of the base polymer, the type pfcrosslinking agent (distances among functional groups) and its amountused, etc. The degree of swelling is determined by the method describedbelow. The same method is used for the working examples described later.

[Determination of Gel Fraction and Degree of Swelling]

A PSA layer sample (weight: W₁) weighing approximately 0.1 g is wrappedinto a pouch with a porous polytetrafluoroethylene membrane (weight: W₂)having an average pore diameter of 0.2 μm, and the opening is tied withtwine (weight: W₃). As the porous polytetrafluoroethylene membrane,trade name NITOFLON® NTF 1122 (product of Nitto Denko Corp.; 0.2 μmaverage pore diameter, 75% porosity, 85 μm thickness) or an equivalentproduct can be used. The resulting package is immersed in 50 mL of ethylacetate and stored at room temperature (typically 23° C.) for 7 days.Subsequently, the package is taken out, and any residual ethyl acetateis wiped off the outer surface. The package weight (W₄) is measured. Thepackage is then dried at 130° C. for 2 hours and the package weight (W₅)is measured. The gel fraction and the degree of swelling of the PSAlayer can be determined by substituting the respective values into thefollowing equation:

Gel fraction(%)=[(W ₅ −W ₂ −W ₃)/W ₁]×100

Degree of swelling (fold)=(W ₄ −W ₂ −W ₃)/(W ₅ −W ₂ −W ₃)

<Surface Properties of PSA Layer>

In the art disclosed herein, the surface of the PSA layer may havespecific properties. The PSA layer according to a preferable embodimenthas a surface hardness of 0.3 MPa or greater. The use of the PSA layerhaving a surface hardness of 0.3 MPa or greater, the applied PSA sheetwill show suppressed aged adhesive strength and provide excellentefficiency of removal from adherend. Increased hardness of the PSA layersurface limits the adhesive strength to or below a certain level andalso limits the increase in adhesive strength with aging; thispresumably suppresses the aged adhesive strength as a whole to a levelthat does not impair the efficiency of removal. In view of the balancebetween suppression of aged adhesive strength and good adhesion, if thesuppression of aged adhesive strength is more important, the surfacehardness of the PSA layer is preferably 0.4 MPa or greater, or morepreferably 0.8 MPa or greater (e.g. 1.2 MPa or greater, or even 2 MPa orgreater). According to the art disclosed herein, even with such arelatively hard PSA which tends to be limited in tightness of adhesionto the adherend, the use of corrosion inhibitor can prevent corrosion ofthe applied surface (covered surface). In this embodiment, the maximumsurface hardness is not particularly limited. From the standpoint of theadhesion, it is usually suitably 5 MPa or less, for instance, 3 MPa orless, or possibly 2.7 MPa or less. In view of the balance, if theprevention of lifting and peeling or the tightness of adhesion toadherend is more important, the surface hardness is preferably less than2 MPa, more preferably less than 1.2 MPa, yet more preferably less than0.8 MPa, or particularly preferably less than 0.4 MPa.

In the embodiment above, the PSA layer is further classified into twoembodiments in view of the duality of suppression of aged adhesivestrength and adhesion to adherend. The PSA layer according to oneembodiment (the first embodiment) has a surface hardness of 0.3 MPa orgreater and less than 0.7 MPa. In this embodiment, the lower limit ofthe surface hardness is preferably 0.32 MPa or greater, more preferably0.34 MPa or greater, or yet more preferably 0.35 MPa or greater; theupper limit is preferably less than 0.55 MPa, more preferably less than0.45 MPa, yet more preferably less than 0.4 MPa, or particularlypreferably less than 0.38 MPa. The PSA layer according to anotherembodiment (the second embodiment) has a surface hardness of 0.7 MPa orgreater and 5 MPa or less. In this embodiment, the lower limit of thesurface hardness is preferably 0.8 MPa or greater, or more preferably0.9 MPa or greater (e.g. 1.2 MPa or greater, or even 2 MPa or greater);the upper limit is preferably 4 MPa or less, more preferably 3 MPa orless, yet more preferably 2.7 MPa or less, or particularly preferably2.5 MPa or less.

The PSA layer according to another preferable embodiment has a surfacehardness of 0.5 MPa or less. With the use of the PSA layer of 0.5 MPa orless in surface hardness, the PSA sheet adheres tightly to the adherendsurface. This effect is combined with the effect of the corrosioninhibitor used to provide excellent corrosion inhibition. In apreferable embodiment, the surface hardness of the PSA layer is lessthan 0.45 MPa, more preferably less than 0.4 MPa (typically less than0.38 MPa, e.g. less than 0.35 MPa), or possibly, for instance, less than0.3 MPa. In this embodiment, the minimum surface hardness is notparticularly limited. It is suitably about 0.1 MPa or greater. From thestandpoint of the light peel and suppression of the increase in agedadhesive strength, it is preferably 0.2 MPa or greater, more preferably0.25 MPa or greater (typically 0.3 MPa or greater, e.g. 0.32 MPa orgreater, or even 0.34 MPa or greater).

The PSA layer according to a preferable embodiment has a slope ofloading curve (the slope of loading curve by nanoindentation,represented by the ratio of the load (μN) to the indentation depth (nm)up to a certain depth) of 0.7×10⁻² μN/nm or greater. The slope (μN/nm)of loading curve provides information equivalent to the elasticity(compressive elasticity) at the PSA layer surface. Suitable elasticitycan suppress the aged adhesive strength and preferably bring aboutexcellent efficiency of removal. The slope of loading curve is morepreferably 0.73×10⁻² μN/nm or greater, yet more preferably 0.75×10⁻²μN/nm or greater, or particularly preferably 0.78×10⁻² μN/nm or greater.According to the art disclosed herein, even with such a relativelyhighly viscoelastic PSA which is limited in tightness of adhesion toadherend, the use of the corrosion inhibitor can prevent corrosion ofthe applied surface (covered surface). In this embodiment, the maximumslope of loading curve is not particularly limited. From the standpointof the adhesion, it is usually 7×10² μN/nm or less, for instance, 5×10⁻²μN/nm or less, or even 4.5×10⁻² μN/nm or less.

The slope of loading curve of the PSA layer according to the firstembodiment is 0.7×10⁻² μN/nm or greater and less than 1.2×10⁻² μN/nm. Inthis embodiment, the minimum slope of loading curve is preferably0.75×10⁻² μN/nm or greater, more preferably 0.8×10⁻² μN/nm or greater(e.g. 0.85×10⁻² μN/nm or greater); the maximum slope is preferably lessthan 1.1×10⁻² μN/nm, more preferably less than 1.0×10⁻² μN/nm, or yetmore preferably less than 0.95×10⁻² μN/nm (e.g. less than 0.8×10⁻²μN/nm). The slope of loading curve of the PSA layer according to thesecond embodiment is 1.2×10⁻² μN/nm or greater and 7×10⁻² μN/nm or less.In this embodiment, the minimum slope of loading curve is preferably1.4×10⁻² μN/nm or greater (e.g. 1.8×10⁻² μN/nm or greater), or even2×10⁻² μN/nm or greater (e.g. 3×10⁻² μN/nm or greater). The maximumslope is preferably 5×10⁻² μN/nm or less, or more preferably 4.5×10⁻²μN/nm or less (e.g. 4×10⁻² μN/nm or less). These numerical ranges aresuitably selected in view of suppression of aged adhesive strength andgood adhesion.

The PSA layer according to another preferable embodiment has a slope ofloading curve less than 1.2×10² μN/nm. From the standpoint of theadhesion to the adherend surface, the slope of loading curve is morepreferably less than 1.1×10⁻² μN/nm, yet more preferably less than1.0×10⁻² μN/nm, or particularly preferably less than 0.95×10⁻² μN/nm(e.g. less than 0.8×10⁻² μN/nm). In this embodiment, the minimum slopeof loading curve is not particularly limited. It is suitably about0.5×10⁻² μN/nm or greater. From the standpoint of providing suitableelasticity to prevent adhesive transfer and the like, it is preferably0.6×10⁻² μN/nm or greater. The slope of loading curve can be 0.7×10⁻²μN/nm or greater, 0.75−10⁻² μN/nm or greater, 0.8×10⁻² μN/nm or greater,or even 0.85×10⁻² μN/nm or greater.

The PSA layer according to a preferable embodiment has a minimum loadbelow 0 μN in the unloading curve by nanoindentation. The decreasingload applied to the indenter during unloading means that it is difficultto pull out the indenter pushed into the PSA layer and further suggeststhat the PSA is cohesive. Such a PSA layer leads to sluggish wetting ofthe adherend surface and the aged adhesive strength is unlikely toincrease. The minimum load of unloading curve is more preferably −0.1 μNor less, for instance, −0.5 μN or less, possibly −1 μN or less, or −2 μNor less. According to the art disclosed herein, even with such arelatively highly cohesive PSA which tends to be limited in tightness ofadhesion to adherend, the use of the corrosion inhibitor can preventcorrosion of the applied surface (covered surface). A low minimum loadof unloading curve may also indicate that the adsorption (adhesion) tothe indenter is being evaluated. In view of the influence of thisfeature on the light peel, the minimum load of unloading curve in thisembodiment is usually suitably −8 μN or greater, preferably −5 μN orgreater, or more preferably −3 μN or greater, for instance, −2 μN orgreater, or −1 μN or greater.

The minimum load of unloading curve of the PSA layer according to thefirst embodiment is −8 μN or greater and less than −1 μN. In thisembodiment, the lower limit of the minimum load of unloading curve ispreferably −5 μN or greater, or more preferably −3 μN or greater; theupper limit is preferably less than −1.5 μN, more preferably less than−1.8 μN, or yet more preferably less than −2 μN. For the PSA layeraccording to the second embodiment, the minimum load of unloading curveis −1 μN or greater and 0 μN or less. In this embodiment, the lowerlimit of the minimum load of unloading curve is preferably −0.5 μN orgreater, more preferably −0.4 μN or greater (e.g. −0.3 μN or greater),or even −0.2 μN or greater (e.g. −0.15 μN or greater). The upper limitis preferably −0.1 μN or less; it can be, for instance, −0.15 μN orless, or −0.2 μN or less.

The PSA layer according to another preferable embodiment has a minimumload of −8 μN or greater in the unloading curve by nanoindentation. Sucha PSA layer readily wet the adherend surface and is likely to adheretightly to the adherend surface. The minimum load of unloading curve ismore preferably −5 μN or greater, or more preferably −3 μN or greater(e.g. −2.5 μN or greater). The minimum load of unloading curve is notparticularly limited. It is suitably about less than 0 μN. Even when thePSA is hard, the indenter pushed into the PSA layer can be easily pulledout and the minimum load of unloading curve can be in a high range (e.g.near 0 μN). In view of these, from the standpoint of obtaining suitablesoftness and wetting properties, the minimum load of unloading curve ispreferably −0.3 μN or less, more preferably −1 μN or less, yet morepreferably −1.5 μN or less, or particularly preferably −2 μN or less(e.g. −2.5 μN or less).

The PSA layer's surface hardness, the slope of loading curve and theminimum load of unloading curve are determined from the load-unload(push-in/pull-out) curve obtained based on nanoindentation, by indentinga nanoindenter from the PSA layer surface (adhesive face) into the PSAlayer up to whichever is less between the depth equivalent to 6% of thethickness of the PSA layer (or the 6% depth; 600 nm for a 10 μm thickPSA layer) and the depth of 300 nm (300 nm depth) (e.g. 300 nm when thePSA layer is 10 μm thick, 120 nm when the PSA layer is 2 μm thick)followed by pulling it out, and plotting the changes in load applied tothe indenter (vertical axis) versus displacement of indenter relative tothe adhesive face (horizontal axis). FIG. 3 schematically illustrates anexample of the load-unload curve by nanoindentation. The surfacehardness (MPa) of the PSA layer is determined by dividing the maximumload (Pmax) (μN) of loading curve by the projected area (A) of contactof the indenter when it is indented up to whichever is less between the6% depth and 300 nm. The slope (μN/nm) of loading curve is determined bydividing Pmax (μN) by the indentation depth (D) (nm). That is, thesurface hardness and the slope of loading curve are determined by thefollowing equations:

Surface hardness (MPa)=Pmax/A

Slope of loading curve (μN/nm)=Pmax(μN)/D(nm)

The minimum load (μN) of unloading curve is the smallest value of theunloading curve.

In the nanoindentation, the measurement is carried out at 300 nm depthor a depth equivalent to 6% of the thickness of the PSA layer. By this,for instance, even with respect to a thin PSA layer as those describedlater in Examples, the behavior of the PSA layer surface can be properlyevaluated without influences of the substrate layer supporting the PSAlayer, etc. With increasing depth, the influence of the supportsubstrate may be detected and the device (typically the indenter size)is also a limiting factor. On the other hand, with respect to a thin PSAlayer, the stress is small at a depth less than 6% and it tends to bedifficult to detect a significant difference. Thus, in the art disclosedherein, whichever is less between the 6% depth and 300 nm is used as themost adequate depth.

The surface hardness, the slope of the loading curve and the minimumload of the unloading curve can be determined by nanoindentation, using,for instance, TRIBOINDENTER available from Hysitron, Inc., under theconditions shown below. The same method is used in the working examplesdescribed later.

[Measurement Conditions]

Indenter used: Berkovich (trigonal pyramid) diamond indenter

Measurement method: Single indentation measurement

Measurement temperature: room temperature (25° C.)

Indentation depth setting: 6% of PSA layer's thickness or 300 nm

Rate of indentation: 100 nm/sec

Rate of removal: 100 nm/sec

The surface hardness, the slope of loading curve and the minimum load ofunloading curve can be adjusted through the base polymer's composition(monomer composition), glass transition temperature (Tg) and molecularweight, the type of crosslinking agent and its amount used, etc. Otherfeatures that can be used for adjusting the properties include the gelfraction as well as the polymerization method and conditions for thebase polymer.

The surface hardness, the slope of loading curve and the minimum load ofunloading curve at 6% depth are referred to as the 6%-depth surfacehardness, the 6%-depth slope of loading curve and the 6%-depth minimumload of unloading curve, respectively. The surface hardness, the slopeof loading curve and the minimum load of unloading curve at 300 nm depthare referred to as the 300 nm-depth surface hardness, the 300 nm-depthslope of loading curve and the 300 nm-depth minimum load of unloadingcurve, respectively. In the art disclosed herein, the surface hardnessencompasses the 6%-depth surface hardness and the 300 nm-depth surfacehardness. The slope of loading curve encompasses the 6%-depth slope ofloading curve and the 300 nm-depth slope of loading curve. The minimumload of unloading curve encompasses the 6%-depth minimum load ofunloading curve and the 300 nm-depth minimum load of unloading curve.

The sulfur atom content in the PSA layer of the PSA sheet disclosedherein is not particularly limited. It can be below about 5000 ppm. Byreducing the sulfide content such as hydrogen sulfide which may causecorrosion, corrosion of the object being protected can be more highlyinhibited. From such a standpoint, the sulfur atom content of the PSAlayer is more preferably below 3000 ppm, or yet more preferably below2000 ppm (e.g. below 1000 ppm). In another preferable embodiment, thesulfur atom content of the PSA layer may be below about 500 ppm, belowabout 300 ppm, or even below about 100 ppm (e.g. below 10 ppm). Thesulfur atom content can be determined from the ratio of elemental sulfurobtained by fluorescent X-ray analysis of the PSA layer using commercialXRF equipment.

<Substrate Layer>

As the substrate layer of the PSA sheet disclosed herein, resin film, arubber sheet, a foam sheet, a composite of these, etc., can be used.Examples of the rubber sheet include natural rubber sheets and butylrubber sheets. Examples of the foam sheet include polyurethane foamsheets, and polychloroprene rubber foam sheets.

The art disclosed herein can be preferably applied to a PSA sheetwherein the substrate layer is resin film. The concept of “resin film”here refers to film typically obtained by molding a thin layer from aresin composition primarily comprising resin components as describedbelow; it should be distinguished from so-called non-woven and wovenfabrics. In other words, the concept of resin film excludes non-wovenand woven fabrics. Preferable resin film is essentially not foamed. Inother words, non-foamed resin film can be preferably used. Here, thenon-foamed resin film refers to resin film that has not beendeliberately subjected to a foaming process. In particular, the resinfilm may have an expansion ratio lower than about 1.1 (e.g. lower than1.05, typically lower than 1.01).

Examples of the resin components forming the resin film includepolyolefinic resins (polyethylene, polypropylene, ethylene-propylenecopolymer, ethylene-vinyl acetate copolymer, etc.), poly(vinylchloride)-based resins (typically soft poly(vinyl chloride)-basedresin); poly(vinyl acetate)-based resin, polyurethane-based resins(ether-based polyurethane, ester-based polyurethane, carbonate-basedpolyurethane, etc.), urethane (meth)acrylate)-based resin, thermoplasticelastomers (olefnic elastomer, styrene-based elastomer, acrylicelastomer, etc.), polyester-based resins (polyethylene terephthalate,polybutylene terephthalate, polyethylene naphthalate, polybutylenenaphthalate, etc.), polycarbonate-based resin, polyamide-based resin,and polyimide-based resin. Among these resins, solely one species or acombination of two or more species can be used.

While no particular limitations are imposed, in the PSA sheet accordingto an embodiment, it is preferable to use a substrate layer thatcomprises, as its primary component(s), one, two or more species ofresin selected from the group consisting of polyolefinic resin,poly(vinyl chloride)-based resin, polyurethane-based resin,thermoplastic elastomer and polyester-based resin (typically a substratelayer comprising such resin in an amount exceeding 50% by weight). Forinstance, a preferable substrate layer comprises one of these resins inan amount exceeding 50% by weight. In another embodiment, in view of theperformance, ease of handling, costs, etc., a substrate layer comprisinga polyolefinic resin layer, polyester-based resin layer or polyvinylchloride-based resin layer can be preferably used. Among the resinmaterials, in view of the heat stability, the lightness of weight, etc.,polyolefinic resins, polyurethane-based resins and olefinic elastomersare preferable; in view of the handling properties, etc., polyolefinicresins and olefnic elastomers are particularly preferable.

The PSA sheet disclosed herein can be preferably made in an embodimentcomprising a substrate layer that comprises a polyolefinic resin as theprimary component, that is, an embodiment wherein the substrate layer ispolyolefinic resin film. For instance, it is preferable to usepolyolefinic resin film in which 50% by weight or more of the entiresubstrate layer is polyethylene (PE) resin or polypropylene (PP) resin.In other words, in the polyolefinic resin film, the combined amount ofPE resin and PP resin may account for 50% by weight or more of theentire substrate layer.

The PP resin may comprise, as the primary component, various polymerspecies (propylene-based polymers) that comprise propylene as a monomerunit. The PP resin may be essentially formed of one, two or more speciesof propylene-based polymer. The concept of propylene-based polymer hereincludes homopolypropylene as well as a random copolymer of propyleneand other monomer(s) (random polypropylene) and a block copolymer (blockpolypropylene). The concept of propylene-based polymer here includes,for instance, the following species:

Propylene homopolymer (homopolypropylene), for instance, isotacticpolypropylene;

Random copolymer (random polypropylene) of propylene and otherα-olefin(s) (typically, one, two or more species selected from ethyleneand α-olefins having 4 to 10 carbon atoms), preferably randompolypropylene comprising propylene as the primary monomer (i.e. themonomer accounting for 50% by weight or more of the total monomercontent);

Block copolymer (block polypropylene) of propylene and other α-olefin(s)(typically, one, two or more species selected from ethylene andα-olefins having 4 to 10 carbon atoms), preferably block polypropylenecomprising propylene as the primary monomer (i.e. the monomer accountingfor 50% by weight or more of the total monomer content).

The PE resin may comprise, as the primary component, various types ofpolymer (ethylene-based polymer) comprising ethylene as a monomer. ThePE resin may be essentially formed of one, two or more species ofethylene-based polymer. The ethylene-based polymer can be an ethylenehomopolymer or a copolymer (random copolymer, block copolymer, etc.) ofethylene as the primary monomer and other α-olefin(s) as secondarymonomer(s). Favorable examples of the α-olefins include α-olefins having3 to 10 carbon atoms such as propylene, 1-butene (which can be abranched 1-butene), 1-hexene, 4-methyl-1-pentene and 1-octene. Forinstance, it is preferable to use PE resin that comprises, as theprimary component, an ethylene-based polymer in which an α-olefin as thesecondary monomer is copolymerized up to about 10% by weight (typicallyup to about 5% by weight).

The PE resin may comprise a copolymer of ethylene and a monomer(functional monomer) containing other functional group(s) in addition toa polymerizable functional group, copolymer of an ethylene-based polymercopolymerized with such a functional monomer, or the like. Examples of acopolymer of ethylene and a functional monomer include ethylene-vinylacetate copolymers (EVA), ethylene-acrylic acid copolymers (EAA),ethylene-methacrylic acid copolymers (EMAA), ethylene-methyl acrylatecopolymers (EMA), ethylene-ethyl acrylate copolymers (EEA),ethylene-methyl methacrylate copolymers (EMMA), and copolymers ofethylene and (meth)acrylic acid (i.e. acrylic acid and/or methacrylicacid) crosslinked by metal ions.

The PE resin is not particularly limited in density. The concept of PEresin here includes all of the following: high density polyethylene(HDPE), medium density polyethylene (MDPE), low density polyethylene(LDPE) and linear low density polyethylene (LLPDE). In an embodiment,the density of the PE resin can be, for instance, about 0.90 g/cm³ to0.94 g/cm³. Preferable PE resins include LDPE and LLDPE. The PE resinmay comprise one, two or more species of LDPE and one, two or morespecies of LLDPE. There are no particular limitations to the respectiveblend ratios of LDPE and LLDPE, or to the LDPE to LLDPE blend ratio.They can be suitably selected to form a PE resin having desirableproperties. As the substrate layer of the PSA sheet disclosed herein, itis preferable to use polyethylenic resin film such as LLDPE film whoseLLDPE content is higher than 50% by weight (preferably about 75% byweight or higher, e.g. about 90% by weight or higher) and LDPE filmwhose LDPE content is higher than 50% by weight (preferably about 75% byweight or higher, e.g. about 90% by weight or higher) Laminate resinfilm comprising such polyethylenic resin film as a component can be usedas well.

The resin film (e.g. polyolefinic resin film) used as the substratelayer of the PSA sheet disclosed herein may comprise, as necessary,suitable components allowable in the substrate layer. Examples ofadditives that can be suitably added include filler, colorant (pigmentsuch as inorganic pigment, dye), antioxidant, photostabilizer (includingradical scavenger and UV absorber), antistatic agent, plasticizer, slipagent, and anti-blocking agent. Each additive can be added, forinstance, in an amount similar to a typical amount in the field of resinfilm used as substrate layers and the like of PSA sheets.

The substrate layer may have a mono-layer structure or a multi-layerstructure formed of two, three or more layers. In a multi-layerstructure, it is preferable that at least one layer (preferably eachlayer) is formed of aforementioned resin film. For instance, in apreferable substrate layer, 75% or more (more preferably 90% or more) ofthe thickness is attributed to mono-layer or multi-layer (typicallymono-layer) polyolefinic resin film. The substrate layer may be entirelyformed of mono-layer or multi-layer polyolefinic resin film. From thestandpoint of the cost-effectiveness, it is preferable to use asubstrate layer formed of mono-layer resin film (e.g. LLDPE film, LDPEfilm, etc.).

The method for producing the substrate layer can be suitably selectedamong heretofore known methods and is not particularly limited. Forinstance, when resin film is used as the substrate layer, it is possibleto use resin film fabricated by suitably employing a heretofore knowngeneral film-forming method such as inflation molding, extrusion, T-diecast molding, and calendar roll molding.

In an embodiment where at least one face (the PSA layer-side face) ofthe substrate layer is a resin film surface, the resin film surface canbe subjected to a heretofore known surface treatment such as coronadischarge treatment, plasma treatment, ozone exposure, flame exposure,UV irradiation, acid treatment, alkali treatment, and primer coating.These surface treatments may enhance the tightness of adhesion betweenthe substrate layer and the PSA layer, or the anchoring of the PSA layeronto the substrate layer. In an embodiment using polyolefinic resin filmas the substrate layer, it is particularly meaningful to provide thesesurface treatments.

<Properties of PSA Sheet>

In the PSA sheet disclosed herein, the initial peel strength is suitablyabout 0.01 N/20 mm or greater, at a tensile speed of 0.3 m/min at 180°peel angle at 30 minutes after applied to a glass plate. The PSA sheetshowing such initial peel strength adheres well to an adherend inrelatively short time and is less likely to lift off the adherend. Whenthe PSA sheet disclosed herein is used as a surface protective sheet, itmay provide good protection. In an embodiment, the initial peel strengthcan be about 0.05 N/20 mm or greater (e.g. about 0.1 N/20 mm orgreater). In an embodiment, the initial peel strength can be about 0.05N/20 mm or greater (e.g. about 0.1 N/20 mm or greater). In a preferableembodiment, the initial peel strength can be about 0.5 N/20 mm orgreater (e.g. about 1 N/20 mm or greater). The maximum initial peelstrength is not particularly limited. From the standpoint of the lightpeel, it is usually suitably about 5 N/20 mm or less, or preferablyabout 2.5 N/20 mm or less (e.g. about 2 N/20 mm or less). The initialpeel strength is determined by the method described below.

[Initial Peel Strength]

The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long stripto prepare a test piece. In a standard environment at 23° C., 50% RH,with a 2 kg rubber roller moved back and forth twice, the test piece ispress-bonded to a glass plate as the adherend. The sample is stored inthe standard environment for 30 minutes. In the same standardenvironment, using a universal tensile tester, the initial peel strength(N/20 mm) is determined at a tensile speed of 0.3 m/min, at 180° peelangle. As the glass plate, a commercial glass plate can be used withoutparticular limitations. For instance, a cut blue plate available fromMatsunami Glass Ind. (1.35 mm thick, 100 mm by 100 mm) or a similarproduct can be used.

In the PSA sheet disclosed herein, there are no particular limitationsto the initial high-rate peel strength P₁, determined at a tensile speedof 30 m/min, at 180° peel angle, at 30 minutes after applied to a glassplate. It is preferably about 10 N/20 mm or less (more preferably about5 N/20 mm or less, e.g. about 3 N/20 mm or less). Such a PSA sheet canbe removed well from an adherend even in an embodiment of use where itis removed in relatively short time after applied to the adherend. Fromthe standpoint of the ease of application to an adherend, protection ofthe adherend in an embodiment used as a surface protective sheet, etc.,the initial high-rate peel strength P₁ is usually suitably about 0.05N/20 mm or greater, preferably about 0.1 N/20 mm or greater, or morepreferably about 0.2 N/20 mm or greater; it can be even 0.5 N/20 mm orgreater (e.g. about 1 N/20 mm or greater). The initial high-rate peelstrength P₁ is measured by the method described below.

[Initial High-Rate Peel Strength P₁]

The tensile speed is changed to 30 m/min. Otherwise in the same manneras the initial peel strength measurement above, the initial high-ratepeel strength P₁ (N/20 mm) can be determined.

In the PSA sheet disclosed herein, there are no particular limitationsto the aged high-rate peel strength P₂, determined at a tensile speed of30 m/min, at 180° peel angle, after applied to a glass plate and storedat 50° C. for seven days. It is preferably less than about 11 N/20 mm.With the PSA sheet satisfying this property, even when it is applied tothe adherend for a relatively long time, the aged adhesive strength issufficiently suppressed and its light peel from adherend can bemaintained. Thus, it shows excellent efficiency of removal fromadherends. With the PSA sheet having an aged high-rate peel strength P₂of about 5 N/20 mm or less (more preferably about 2 N/20 mm or less),greater efficiency of removal can be obtained. In an embodiment, theaged high-rate peel strength P₂ can be about 1 N/20 mm or less, or evenabout 0.5 N/20 mm or less. From the standpoint of inhibiting lifting andpeeling while the adherend is protected (e.g. during processing of theadherend with the PSA sheet applied thereon), the aged high-rate peelstrength P₂ is usually suitably about 0.05 N/20 mm or greater,preferably about 0.1 N/20 mm or greater, or more preferably about 0.3N/20 mm or greater. The aged high-rate peel strength P₂ is determined bythe method described below.

[Aged High-Rate Peel Strength P₂]

The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long stripto prepare a test piece. In a standard environment at 23° C., 50% RH,with a 2 kg rubber roller moved back and forth twice, the test piece ispress-bonded to a glass plate as the adherend. The sample is stored inan environment at 50° C. for seven days and then in a standardenvironment at 23° C., 50% RH for one hour. Subsequently, in the samestandard environment, using a universal tensile tester, the agedhigh-rate peel strength P₂ (N/20 mm) is determined at a tensile speed of30 m/min, at 180° peel angle. The glass plate used as the adherend isthe same as the one used in the initial peel strength measurement.

In the PSA sheet disclosed herein, the increase in aged adhesivestrength (i.e. P₂-P₁, the difference of aged high-rate peel strength P₂(N/20 mm) and the initial high-rate peel strength P₁ (N/20 mm)) is notparticularly limited. It is preferably 8.5 N/20 mm or less. A limitedincrease in aged adhesive strength may suggest, in addition tosuppression of the increase in aged adhesive strength, the absolutevalue of the aged adhesive strength is limited to a level that does notcompromise the efficiency of removal when the initial adhesive strengthis limited. The PSA sheet satisfying this property is likely to provideexcellent efficiency of removal. The increase (P₂-P₁) in aged adhesivestrength is more preferably 5 N/20 mm or less, yet more preferably 3.5N/20 mm or less, or particularly preferably 1 N/20 mm or less (typically0.5 N/20 mm or less, e.g. 0.2 N/20 mm or less). Although the increase(P₂-P₁) in aged adhesive strength is usually 0 N/20 mm or greater, thePSA sheet disclosed herein is not limited to a kind that will alwayshave increased aged adhesive strength and P₂-P₁ can be, for instance,about −3 N/20 mm or greater, about −1 N/20 mm or greater, about −0.5N/20 mm or greater, about 1 n/20 mm or greater, or about 3 N/20 mm orgreater (e.g. about 5 N/20 mm or greater).

While no particular limitations are imposed, the PSA sheet disclosedherein may have a ratio of aged high-rate peel strength P₂ (N/20 mm) toinitial high-rate peel strength P₁ (N/20 mm) (i.e. a P₂/P₁ ratio value)of 5 or lower. A small P₂/P₁ ratio value indicates a small increase inpeel strength with aging. By this, initial adhesion and light peelduring removal are favorably combined. From such a standpoint, the P₂/P₁ratio is preferably 4 or lower, more preferably 3 or lower, or yet morepreferably 2 or lower, for instance, 1.8 or lower, 1.5 or lower, or even1.3 or lower. The P₂/P₁ ratio is typically 0.8 or higher; it can be, forinstance, 1 or higher.

The PSA sheet disclosed herein may have a level of corrosion inhibitiongraded as good in the corrosion inhibition test carried out by themethod described later in Examples. The PSA sheet satisfying thisproperty is preferably used for an application involving adhesion tovarious types of adherend containing corrosive substances.

<Applications>

The PSA sheet disclosed herein is preferably used as a surfaceprotective sheet that is to be applied to surfaces of a metal plate, acoated steel plate, a synthetic resin plate, a glass plate and the likeso as to prevent damage (scratches, contamination, etc.) to thesesurfaces while they are being processed or transported and to beeventually removed from the adherend at the end of the protectionperiod. The PSA sheet can prevent corrosion of a surface being protectedand thus is preferably used as a surface protective sheet on an articlewhose surface or interior includes a corrosion-sensitive substance suchas silver.

The PSA sheet disclosed herein is favorable as a surface protectivesheet for a glass plate used as a building material such as windowglass, etc. The glass plate subject to application (protection)typically comprises a glass substrate and a coating layer placed on theglass substrate, wherein the coating layer may include a metal layer.More specifically, the glass plate may have a Low-E layer on one face.The Low-E layer usually includes a layer of metal such as silver. Inproducing the glass plate, the Low-E layer surface may be left exposeduntil two glass plates including the Low-E-layer-bearing glass plate areassembled into a pair-glass (e.g. dual-pane glass) with theLow-E-layer-side surface on the inside. The PSA sheet disclosed hereinis preferably used to prevent the Low-E layer surface not only fromdamage, degradation, wearing, etc., but also from corrosion. In otherwords, the PSA sheet can be used as a protective sheet for a Low-E layersurface. Low-E-layer-bearing glass plates have higher levels of heatblocking or thermal insulation as compared to conventional glass platesand can improve the efficiency to cool down or heat up indoor spaces;and therefore, they are widely used as building materials such as windowglass. The art disclosed herein may indirectly contribute to energysaving and reduction of greenhouse gas emissions.

From the standpoint of the efficiency of removal, the PSA sheetdisclosed herein is preferably used on an adherend having a largesurface area on which the peel strength tends to be limited. The PSAsheet disclosed herein is preferably used in an embodiment where itcovers the entire surface of an adherend having a width of about 1 m orgreater, for instance, about 2 m or greater (or even about 3 m orgreater). The length of the adherend surface is equal to or greater thanthe width. In a preferable embodiment, it is preferably used in anembodiment where it entirely covers the surface of one face of a largeflat plate (favorably, a flat plate with a smooth surface). Inparticular, glass plates used for building materials such as windowglass are becoming progressively larger in view of efficient production,transportation, etc. It is preferably used in an embodiment where itcovers the entire surface of a glass plate (typically the entire Low-Elayer surface of the Low-E-layer-bearing glass plate) having a largesurface area (e.g. with a surface width above 2.6 m or even at or aboveabout 3 m). The art disclosed herein can bring about great corrosioninhibition while maintaining light peel with respect to an adherendhaving such a large surface area.

The PSA sheet according to an embodiment disclosed herein may showsuppressed aged adhesive strength and thus may provide good efficiencyof removal even when, for instance, the period of adhesion to theadherend (which can be the protection period for the adherend) becomesrelatively long (typically two weeks or longer, e.g. four weeks orlonger). Thus, for instance, it can be favorably used in an embodimentof use where the period from application to adherend (e.g. a glassplate) up to removal from the adherend can be two weeks or more (e.g.four weeks or more).

Matters disclosed by this description include the following:

-   (1) A method for producing a glass unit, the method comprising:

a step (A) of obtaining a glass plate comprising a glass substrate and aLow-E layer placed on the glass substrate;

a step (B) of applying a protective sheet to the Low-E layer surface ofthe glass plate;

an optional step (C) of subjecting the glass plate to at least oneprocess selected from the group consisting of transportation, storage,processing, washing and handling;

a step (D) of removing the protective sheet from the glass plate; and

a step (E) of assembling a glass unit using the glass plate;

wherein the protective sheet comprises a substrate layer, and a PSAlayer formed on at least one face of the substrate layer, and

the PSA layer comprises, as a corrosion inhibitor, a surfactant free ofsulfur atoms.

-   (2) The method according to (1) above, wherein the glass plate has a    width of 1 m or greater.-   (3) The method according to (1) above, wherein the glass plate has a    width of 2 m or greater.-   (4) The method according to (1) above, wherein the glass plate has a    width greater than 2.6 m.-   (5) The method according to (1) above, wherein the glass plate has a    width of 3 m or greater.-   (6) The method according to any of (1) to (5) above, wherein the    Low-E layer comprises a metal layer.-   (7) The method according to any of (1) to (5) above, wherein the    Low-E layer comprises a silver layer.-   (8) The method according to any of (1) to (7) above, wherein the    Low-E layer has a thickness of 1000 nm or less.-   (9) The method according to any of (1) to (8) above, wherein the    step (B) includes a step of entirely covering one face of the glass    plate with at least one of the protective sheet.-   (10) The method according to any of (1) to (9) above, wherein the    step (C) is essential and in the step (C), the glass plate is washed    using water.-   (11) A method for protecting an article surface, the method    comprising

a step of applying a protective sheet to a surface of an article before,during or after processing (an application step), wherein the article'ssurface or interior comprises a corrosive substance,

wherein the protective sheet comprises a substrate layer and a PSA layerprovided to at least one face of the substrate layer, and

the PSA layer comprises, as a corrosion inhibitor, a surfactant free ofsulfur atoms.

-   (12) The method according to (11) above, further comprising a step    of removing the protective sheet from the article (a removal step)    and optionally including, between the application step and the    removal step, at least one process selected from the group    consisting of transporting, storing, processing, washing and    handling the article having the protective sheet applied thereon.-   (13) The method according to (11) or (12) above, wherein the article    comprises a glass substrate and a coating layer placed on the glass    substrate, with the coating layer including a metal layer.-   (14) The method according to any of (11) to (13) above, wherein the    article is a glass plate having a Low-E layer on one face, and

the application step includes a step of applying the protective sheet tothe face of the glass plate on which the Low-E layer is formed.

-   (15) The method according to (14) above, wherein the glass plate has    a width of 1 m or greater, and

the application step includes a step of entirely covering one face ofthe glass plate with at least one of the protective sheet.

-   (16) The method according to any of (1) to (15) above, wherein the    corrosion inhibitor is an anionic surfactant or a nonionic    surfactant.-   (17) The method according to any of (1) to (16) above, wherein the    corrosion inhibitor is a phosphate represented by the formula (a):

-   ; (in the formula (a), R¹ is —OH or —(OCH₂CH₂)_(n)OR³; R² represents    —(OCH₂CH₂)_(m)OR⁴; n and m are identical or different and each is an    integer between 1 and 30; R³ and R⁴ are identical or different and    each is an organic group with 1 to 30 carbon atoms).-   (18) The method according to any of (1) to (17) above, wherein the    corrosion inhibitor content in the PSA layer is 0.05 part to 10    parts by weight to 100 parts by weight of the base polymer in the    PSA layer.-   (19) The method according to any of (1) to (18) above, wherein the    PSA layer has a surface hardness of 0.3 MPa or greater.-   (20) The method according to any of (1) to (19) above, wherein the    PSA layer has a surface hardness of 0.5 MPa or less.-   (21) The method according to any of (1) to (20) above, wherein the    PSA layer is formed from a water-dispersed PSA composition, a    solvent-based PSA composition or a hot-melt PSA composition.-   (22) The method according to any of (1) to (21) above, wherein the    PSA layer is formed from a water-dispersed PSA composition.-   (23) The method according to any of (1) to (22) above, wherein the    PSA layer is an acrylic PSA layer comprising an acrylic polymer as    its base polymer or a rubber-based PSA layer comprising a    rubber-based polymer as its base polymer.-   (24) The method according to any of (1) to (23) above, wherein the    protective sheet shows an initial peel strength to a glass plate is    5 N/20 mm or less.-   (25) The method according to any of (1) to (24) above, wherein the    sulfur content in the PSA layer is less than 2000 ppm.-   (26) A PSA sheet comprising a substrate layer and a PSA layer    provided to at least one face of the substrate layer, wherein the    PSA layer comprises, as a corrosion inhibitor, a surfactant free of    sulfur atoms.-   (27) The PSA sheet according to (26) above, wherein the corrosion    inhibitor is an anionic surfactant or a nonionic surfactant.-   (28) The PSA sheet according to (26) or (27) above, wherein the    corrosion inhibitor is a phosphate represented by the formula (a):

-   ; (in the formula (a), R¹ is —OH or —(OCH₂CH₂)_(n)OR³; R² represents    —(OCH₂CH₂)_(m)OR⁴; n and m are identical or different and each is an    integer between 1 and 30; R³ and R⁴ are identical or different and    each is an organic group with 1 to 30 carbon atoms).-   (29) The PSA sheet according to any of (26) to (28) above, wherein    the corrosion inhibitor content in the PSA layer is 0.05 part to 10    parts by weight to 100 parts by weight of a base polymer in the PSA    layer.-   (30) The PSA sheet according to any of (26) to (29) above, wherein    the PSA has a surface hardness of 0.3 MPa or greater.-   (31) The PSA sheet according to any of (26) to (30) above, wherein    the PSA layer has a surface hardness of 0.5 MPa or less.-   (32) The PSA sheet according to any of (26) to (31) above, wherein    the PSA layer is formed from a water-dispersed PSA composition, a    solvent-based PSA composition or a hot-melt PSA composition.-   (33) The PSA sheet according to any of (26) to (32) above, wherein    the PSA layer is formed from a water-dispersed PSA composition.-   (34) The PSA sheet according to any of (26) to (33) above, wherein    the PSA layer is an acrylic PSA layer comprising an acrylic polymer    as its base polymer or a rubber-based PSA layer comprising a    rubber-based polymer as its base polymer.-   (35) The PSA sheet according to any of (26) to (34) above, showing    an initial peel strength of 5 N/20 mm or less to a glass plate.-   (36) The PSA sheet according to any of (26) to (35) above, wherein    the sulfur content in the PSA layer is less than 2000 ppm.-   (37) A surface protective sheet comprising the pressure-sensitive    adhesive sheet according to any of (26) to (36) above, wherein the    PSA sheet comprises the PSA layer provided to one face of the    substrate layer.-   (38) Use of a surfactant free of sulfur atoms as a corrosion    inhibitor in PSA.-   (39) The use according to (38) above, wherein the corrosion    inhibitor is a phosphate represented by the formula (a):

-   ; (in the formula (a), R¹ is —OH or —(OCH₂CH₂)_(n)OR³; R² represents    —(OCH₂CH₂)_(m)OR⁴; n and m are identical or different and each is an    integer between 1 and 30; R³ and R⁴ are identical or different and    each is an organic group with 1 to 30 carbon atoms).-   (40) The use according to (38) or (39) above, wherein the corrosion    inhibitor is present on a surface of an area in which an object    protected from corrosion is present.-   (41) The use according to (40) above, wherein the object protected    from corrosion is a metal.

EXAMPLES

Several working examples related to the present invention are describedbelow, but the present invention should not be limited to theseexamples. In the description below, “part(s)” and ‘%’ are by weightunless otherwise specified.

Example 1

Were mixed 58 parts of butyl acrylate (BA), 40 parts of n-butylmethacrylate (BMA), 2 parts of acrylic acid (AA), 3 parts of emulsifier(based on non-volatiles), and 42 parts of ion-exchanged water. Theresulting mixture was stirred with a homomixer to prepare an aqueousemulsion (monomer emulsion). As the emulsifier, was used a reactiveanionic emulsifier (trade name AQUALON KH-1025, product of Dai-ichiKogyo Seiyaku Co., Ltd.).

Into a reaction vessel equipped with a condenser, a nitrogen inlet, athermometer and a stirrer, was placed 51.5 parts of ion-exchanged water;the resultant was stirred at room temperature for at least one hourwhile introducing nitrogen gas. Subsequently, the system was heated to70° C.; 0.05 part of ammonium persulfate was added; and while stirring,the monomer emulsion was gradually added over three hours. After thecompletion of the addition of the monomer emulsion, the reaction mixturewas continuously stirred at 75° C. for two hours and then cooled to 30°C. To the resulting polymerization reaction mixture, 10% ammonia waterwas added to adjust the pH to 8. An aqueous dispersion of acrylicpolymer (50% acrylic polymer) according to this Example was thusobtained.

To the resulting aqueous acrylic polymer dispersion, for 100 parts ofthe acrylic polymer therein, were added 1 part of a crosslinking agentand 0.6 part of a corrosion inhibitor. The resulting mixture was stirredat 23° C. at 300 rpm for 10 minutes to prepare a PSA compositionaccording to this Example. As the crosslinking agent, was used anepoxy-based crosslinking agent (trade name TETRAD-C,1,3-bis(N.N-diglycidylaminomethyl)cyclohexane, product of Mitsubishi GasChemical, Inc.). As the corrosion inhibitor, was used polyoxyethylenealkyl ether phosphate (product of Toho Chemical Industry Co., Ltd.,product name PHOSPHANOL RS-410).

Two sheets of 50 μm thick resin film formed of LDPE having onecorona-treated face were obtained. The acrylic PSA composition wasapplied to the corona-treated face of the first sheet of resin film andallowed to dry at 90° C. for 1 minute to form a 5 μm thick PSA layer andobtain a PSA sheet having an overall thickness of 55 μm. Thenon-corona-treated face (second face) of the second sheet of resin filmwas applied to the PSA layer and used as release liner.

Example 2

The crosslinking agent was used in the amount shown in Table 1.Otherwise in the same manner as Example 1, a PSA composition accordingto this Example was prepared and a PSA sheet was fabricated.

Example 3

No corrosion inhibitor was used. The monomer composition and thecrosslinking agent content were as shown in Table 1. Otherwise basicallyin the same manner as Example 1, were prepared a PSA composition and aPSA sheet.

[Corrosion Inhibition Test]

The PSA sheet to be measured was cut to a 20 mm wide by 100 mm longstrip to prepare a test piece. In a standard environment at 23° C., 50%RH, with a 2 kg rubber roller moved back and forth twice, the test piecewas press-bonded to the Low-E layer surface of a Low-E-layer-bearingglass plate as the adherend. As the Low-E-layer-bearing glass plate, wasused a product of Nippon Sheet Glass Company, Ltd. under item numberRSFL6AL (6 mm thick, 100 mm by 100 mm). The sample was stored in anenvironment at 40° C. and 92% RH for seven days. With respect to thearea protected with the PSA sheet, the surface condition of the glassplate was visually inspected. When no corrosion was observed, it wasgraded “Good.” When some corrosion was observed, it was graded “Poor.”The results are shown in Table 1.

As the adherend, any glass plate bearing a silver-layer-containing Low-Elayer can be used without particular limitations. An item similar to theproduct above or any other commercial Low-E-layer-bearing glass platemay be used.

For each Example, by the methods described above, were determined thesurface hardness (MPa), the slope (μN/nm) of loading curve, and theminimum load (μN) of unloading curve. The results are shown in Table 1along with the PSA composition for each Example.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Monomer composition 2EHA — 92 (parts) BA 58 —BMA 40 — MMA — 4 AA  2 4 Tg −28° C. −62° C. Corrosion inhibitorPhosphate 0.6 0.6 — Crosslinking agent (parts) Epoxy 1 3 6 Surfacehardness (MPa) 0.286 0.353 0.945 Slope of loading curve (μN/nm) 0.00670.0077 0.0155 Minimum load of unloading curve (μN) −4.33 −4.51 −0.295Corrosion inhibition Good Good Poor

As shown in Table 1, with respect to Examples 1 and 2 using PSAcontaining a sulfur-free surfactant as the corrosion inhibitor, nocorrosion was observed on the glass plate surface. On the other hand, inExample 3 where the corrosion inhibitor was not used, corrosion wasfound in the silver layer on the glass plate surface in the corrosioninhibition test. These results indicate that according to a PSA sheetthat comprises a PSA layer including a sulfur-free surfactant as thecorrosion inhibitor, corrosion of the surface being protected can beprevented.

Although specific embodiments of the present invention have beendescribed in detail above, these are merely for illustrations and do notlimit the scope of the claims The art according to the claims includesvarious modifications and changes made to the specific embodimentsillustrated above.

REFERENCE SIGNS LIST

-   1: substrate layer-   1A: first face-   1B: second face-   2: PSA layer-   2A: adhesive face-   10: PSA sheet-   100: glass plate (Low-E-layer-bearing glass plate)-   110: glass substrate-   120: Low-E layer-   200: protective sheet-   300: glass unit-   320: another glass unit-   340: spacer

What is claimed is:
 1. A method for producing a glass unit, the methodcomprising: a step (A) of obtaining a glass plate comprising a glasssubstrate and a Low-Emissivity layer placed on the glass substrate; astep (B) of applying a protective sheet to the Low-Emissivity layersurface of the glass plate; a step (C) of subjecting the glass platewith the protective sheet applied thereon to at least one processselected from the group consisting of transportation, storage,processing, washing and handling; a step (D) of removing the protectivesheet from the glass plate; and a step (E) of assembling a glass unitusing the glass plate; wherein, the protective sheet comprises asubstrate layer and a pressure-sensitive adhesive layer on at least onesurface of the substrate layer, and the pressure-sensitive adhesivelayer comprises, as a corrosion inhibitor, a surfactant free of sulfuratoms.
 2. The method according to claim 1, wherein the glass plate has awidth of 1 m or greater.
 3. The method according to claim 1, wherein theglass plate has a width of 2 m or greater.
 4. The method according toclaim 1, wherein the glass plate has a width greater than 2.6 m.
 5. Themethod according to claim 1, wherein the glass plate has a width of 3 mor greater.
 6. The method according to according to claim 1, wherein theLow-Emissivity layer includes a metal layer.
 7. The method according toaccording to claim 1, wherein the Low-Emissivity layer includes a silverlayer.
 8. The method according to according to claim 1, wherein theLow-Emissivity layer has a thickness of 1000 nm or less.
 9. The methodaccording to according to claim 1, wherein the step (B) includescovering an entire surface of the glass plate with at least one of theprotective sheet.
 10. The method according to according to claim 1,wherein in the step (C), the glass plate is washed using water.
 11. Apressure-sensitive adhesive sheet comprising a substrate layer and apressure-sensitive adhesive layer on at least a surfaceof the substratelayer, wherein the pressure-sensitive adhesive layer comprises, as acorrosion inhibitor, a surfactant free of sulfur atoms.
 12. Thepressure-sensitive adhesive sheet according to claim 11, wherein thecorrosion inhibitor is an anionic surfactant or a nonionic surfactant.13. The pressure-sensitive adhesive sheet according to claim 11, whereinthe corrosion inhibitor is a phosphate represented by a formula (a):

(in the formula (a), R¹ is —OH or —(OCH₂CH₂)_(n)OR³; R² represents—(OCH₂CH₂)_(m)OR⁴; n and m are identical or different and each is aninteger between 1 and 30; R³ and R⁴ are identical or different and eachis an organic group with 1 to 30 carbon atoms).
 14. Thepressure-sensitive adhesive sheet according to claim 11, wherein thecorrosion inhibitor content in the pressure-sensitive adhesive layer is0.05 part to 10 parts by weight to 100 parts by weight of a base polymerin the pressure-sensitive adhesive layer.
 15. The pressure-sensitiveadhesive sheet according to claim 11, wherein the pressure-sensitiveadhesive layer has a surface hardness of 0.3 MPa or greater.
 16. Thepressure-sensitive adhesive sheet according to claim 11, wherein thepressure-sensitive adhesive layer has a surface hardness of 0.5 MPa orless.
 17. The pressure-sensitive adhesive sheet according to claim 11,wherein the pressure-sensitive adhesive layer is formed from awater-dispersed pressure-sensitive adhesive composition, a solvent-basedpressure-sensitive adhesive composition or a hot-melt pressure-sensitiveadhesive composition.
 18. The pressure-sensitive adhesive sheetaccording to claim 11, wherein the pressure-sensitive adhesive layer isformed from a water-dispersed pressure-sensitive adhesive composition.19. The pressure-sensitive adhesive sheet according to claim 11, whereinthe pressure-sensitive adhesive layer comprises an acrylic polymer asits base polymer or a rubber-based polymer as its base polymer.
 20. Thepressure-sensitive adhesive sheet according to claim 11, wherein thepressure-sensitive adhesive layer has a thickness of 15 μm or less.